Scrolls and Potions vs. Wand
For an ongoing campaign, at least, scrolls and particularly potions would not be a good choice. They’re very expensive, and will consume enormous amounts of your wealth if you’re using them a lot. They’re also nearly unusable mid-combat due to the action costs of getting one out of a pack and using it. So usually, you are much better off with a wand of cure light wounds that you can zap between combats to top off hp totals. At 750 gp, this is quite affordable for a 4th-level character, though really your party should contribute. It’s also the best hp:gp ratio in the game aside from a wand of infernal healing, which will cause problems for a good-aligned cleric.
In a one-shot, the wand vs. scroll comparison is much less clear. (Not so much potions; potions are incredibly overpriced.) Normally, you can be sure you’ll go through 50 charges of cure light wounds, and so the wand costs 15 gp per casting while each scroll of cure light wounds costs 25 gp. But the wand only starts becoming economically favorable if you use 38 or more charges. So definitely buy a wand of cure light wounds before you buy 38 scrolls of cure light wounds, but it’s not clear here that you would buy 38.
That said, I still think you should go with the wand. The overwhelming majority of your healing should come from these wands, and you don’t want to skimp. Just make sure you’re at full hp after every fight—particularly if you know you need to use 38 charges to get your money’s worth, there’s little incentive to be stingy. And you want to be topped off, because you do not want to heal in the middle of a fight, and you don’t want to burn spell slots on healing either. You should use spell slots for other, more powerful things, and channel energy should be reserved for emergencies (i.e. you’re still in the middle of a fight and someone is bleeding out).
If you do go through a lot of charges, and the one-shot involves leveling up, and if access to places to buy replacement wands of cure light wounds are a problem, consider Craft Wand at 5th—at 750 gp, you can finish a wand of cure light wounds in a single evening.
(Scrolls to rectify rarer conditions—stuff like a scroll of remove curse—are still probably a good idea. Don’t ever buy potions for any reason, especially not as a spellcaster who can use a scroll for the same effect but much more cheaply.)
Ability Scores and Weapons
Using a wand for healing also means you can skimp on Charisma—and since you want to be tanky, you really should, because you need too many other ability scores. Your Wisdom needs to be a 14, or 16 if you expect to reach 5th level—you want all the bonus spell slots you can use (well, Wis 20 could get a second 1st-level bonus spell slot—but that’s too much for too little). After that, you probably want to focus on Dexterity and Constitution.
I highlight “want” above because unfortunately, focusing on Dexterity doesn’t work out very well for you. Focusing on Dexterity from the front line means Weapon Finesse, and some kind of finesse-able weapon. Unfortunately, being a front line basically requires reach, or else it is all too easy for enemies to just go around you and attack your allies that way. Combining these two properties is... not easily achieved, at least as a cleric, at least at your level, at least with your sources. Ordinarily, effortless lace cheaply solves the problem. But that’s from Giant Hunter’s Handbook, which isn’t on your list. If there was a deity that had an elven branched spear as a favored weapon, you could use that, but there isn’t (at least on the official pantheon). And at higher levels, righteous might is a fantastic cleric spell for size and reach, or you could burn a ton of feats for Improved Whip Mastery, but you’re 3 levels away from accessing either of those.
There isn’t really a good solution here. You could go with Guided Hand instead of Weapon Finesse, but that requires the otherwise-terrible Channel Smite feat, and there go both of your feats unless you’re human. And frankly, since this is a one-shot, you’d prefer to use Dexterity over Wisdom, since Wisdom isn’t going to get you any more spells unless you level up some. So you’d be better off with Exotic Weapon Proficiency (elven branched spear) and Weapon Finesse, which is rather painful. Ultimately, you’re probably going to have to just split the difference and have decent Strength and at least not dump Dexterity.
You probably want to be human. You can get +2 to Strength or Constitution, or Dexterity if you go that route, and you get a bonus feat, which—it’s hard to overstate how good that bonus feat is. It is so, so good. You’re strapped for ability scores, so +2 Con when you already have +2 Str or +2 Dex might be tempting, but the bonus feat is probably better.
If you had effortless lace and could finesse something with reach without burning a feat on Exotic Weapon Proficiency, the caligni would be a consideration, with +2 Dex, +2 Con, and −2 Int that we don’t care about, plus the very-good see in darkness ability (that is, including magical darkness, unlike darkvision). The main drawback—being dazzled in bright light—barely matters because dazzled is an extremely minor condition (unlike blindness, even temporary, that other light-sensitive creatures can suffer from). But since you’ll have to attack with Strength, you won’t see as much benefit.
Unfortunately, there doesn’t appear to be any Str + Con race available that isn’t considered higher-power. So really, human seems to be the way to go.
I will note, however, that if you’re allowed more powerful, “monstrous” races, the centaur is an incredibly good choice with +6 Strength, +2 Constitution, and +2 Wisdom—that right there makes the whole concern about Weapon Finesse moot, since you’d be able to afford Strength and Dexterity (or just get a huge Strength and enjoy extra-high accuracy). Of course, it also doesn’t come with a bonus feat which means you don’t really come out “ahead” on feats compared to a human with effortless lace, but since you don’t have access to effortless lace that’s a moot point.
For domains, I looked into those you were looking at, and the subdomains of those, and also the Repose domain, which I was looking at because for a while I was thinking of recommending Selket as your deity (because I thought her favored weapon, the scorpion whip, was a reach, trip, light weapon—but sadly it is not unless you use it as a whip, at which point is loses the ability to threaten and becomes worthless).
Good: Touch of good is a fine effect, but the standard action to use it and 1-round duration are extremely limiting, since that means you have to spend your turn using it on someone the exact same round they’re going to need it.
- Friendship: Friendship offers unparalleled communication for the level, which can be a really big deal. Depends a lot on the nature of the campaign, though. Still, the improvement in the action economy means a whole lot. Anyway, I’m not sure Iomedae offers Friendship anyway.
War: Battle rage is pretty awful. Half your level isn’t a huge damage bonus, and you have to spend an entire turn giving it. Best case scenario (20th level, targeting an dual-wielding ally with 7 attacks) it’s 70 damage—which isn’t bad, but that assumes all of those attacks hit, which isn’t a reasonable assumption at all. And even then it’s still “not bad.” If you target someone who isn’t dual-wielding, you’re looking at 40 damage at most—not a great use of your turn at all. (Besides, if a melee type can full-attack a target like this, they can almost-certainly kill it even without this bonus, so it’s also likely to be overkill.)
- Tactics: Giving an ally the better of two initiative rolls is awesome, and much better than the lousy battle rage. In many combats, initiative is the most important roll that gets made. And Iomedae definitely offers Tactics.
Repose: For more offensive support, staggered is a pretty brutal effect, and there’s no save. That’s quite strong. Iomedae doesn’t offer Repose, though, if you’re using those rules.
Personally, I like Repose and Tactics of these. Good and Tactics, or Friendship and Tactics, is fine, though. Good and War, I don’t really recommend. If deity is a concern, Repose and Tactics is available through Acavna (though the wiki entry says she is “long-forgotten” so that might not be legit), and Friendship and Tactics is available through Chaldira Zuzarista (who is primarily a halfling deity; these domains are not a good reason to choose halfling here, but I think she has non-halfling deities). Good and Tactics are a valid choice for Iomedae.
Ultimately, actions in combat are precious, precious things. There is just a very limited window of opportunity to change the outcome of a fight. Each turn can potentially do a ton, and after a few of them, battles start getting pretty lop-sided one way or the other. It’s a rare Pathfinder combat that lasts more than 3 rounds—and even if mopping up or effecting a retreat still need to happen, the actual winner and loser of the fight is often decided long before then.
In a fight, generally, the best way to support the party is to end the fight. Healing is too little, too late—finish the fight, then worry about topping off. Even buffing, mid-combat, is painful—this is why I was down on touch of good and battle rage. You want buffs you can cast ahead of time, so that you don’t have to spend precious actions in combat buffing when you could be using those buffs. So look for buffs which last a decent amount of time. Look for buffs that can hit the entire party with a single casting—that saves slots but more importantly actions if you have to use it in combat. And look for spells that will allow you to buff more effectively, for example by scouting. Divinations, so you know what buffs you’ll need when, are a great idea.
Improved Initiative is good—again, initiative is often the most important roll in a combat. But it’s only good, not amazing. You have very, very few feats to work with, and I think you can do better.
Weapon Focus, on the other hand, is awful—and more to the point, cheaply bought by putting a cracked opalescent white pyramid ioun stone into a wayfinder. Even if those aren’t available in your campaign, the mere fact that the feat is available for a trivial 2,000 gp is a sign how poor it is. You don’t need or want it.
Instead, consider “real” combat feats. You plan to stand in front of enemies to protect the backline, but you can’t do that without battlefield presence. A guisarme with Combat Reflexes and Improved Trip is a fairly effective way to go. Dirty Trick is another good combat maneuver, since it has a lot of options. The Dirty Fighting feat is a convenient way to qualify for all of these.
If you don’t go that way, consider Eldritch Heritage, for getting some juicy sor/wiz spells—the sor/wiz list is very strong (not that the cleric’s is bad by any means), so you can get a lot of mileage out of that. And again, if you reach 5th, consider the aforementioned Craft Wand. Actually, for that matter, Craft Wondrous Item is a pretty strong feat for 3rd even if you have decent access to ye olde magic item shoppe—if your GM will allow you to start with more magic items since your cleric has had Craft Wondrous Item since 3rd, and your party has been together so you could have craft things for your allies at a 50% discount, this is incredible.
Your Wisdom should definitely be 14, unless you expect to see 5th level, in which case it should be 16. This stays consistent no matter what else you do.
Personally, I’d probably go with a human, with Weapon Finesse, Combat Reflexes, and Exotic Weapon Proficiency (elven branched spear). Then your non-Wisdom ability scores could be Dex > Con ≫ everything else.
The Strength-based route goes for Dirty Fighting, Improved Trip, and Combat Reflexes, and uses a guisarme. The advantage is tripping; the disadvantage is lack of Dexterity, and also probably not having as high accuracy or CMB because your Strength probably won’t be as high as the elven branched spear version’s Dexterity.
In both cases, I’d ask about how Craft Wondrous Items might affect the party’s initial load-out of wondrous items. If every wondrous item in the entire party is half off, that would be worth ditching Combat Reflexes for. Even if the GM isn’t that generous, depending on how generous they are, it may still be worthwhile, particularly for a Strength-based version that has low Dexterity.
If you skip Combat Reflexes, I would strongly recommend the Decisive Strategist trait if your game uses traits and isn’t strictly requiring Golarion faith. (In Golarion, this requires worship of Torag—a friend of Iomedae’s, but not Iomedae herself. If you are bound by this requirement, this will mess with your domains—Torag doesn’t offer any of the domains we were looking at.)