If someone was to use their coins as thrown weapons, no sling just out of hand, how would you roll the attack and damage rolls? Is there a damage chart for something like this?
The question makes an assumption of D&D4 that isn't really valid. Unlike previous editions of D&D weapons do not deal damage .... directly.
That means that the answer to your question is composed of:
What kind of weapon is the coin?
By the book, the answer is an Improvised Ranged Weapon. Which states that it covers "anything you happen to pick up, from a rock to a chair." This gives us a weapon with 1d4 damage and a range of 5/10.
Like anything, this could be overruled by the DM. Judging from the other answers and comments there is a lot of sentiment for this. If this is a one time thing, I'd strongly encourage the DM to not create a house rule on the fly. Let the action flow, the player is probably trying to get out of a sticky situation! You'll also dive down the rabbit hole of having to answer exactly how heavy something needs to be to qualify as a weapon. Would a sack of 5 coins be enough? How about a lantern?
Recognize that the player is already handicapping himself by using an improvised weapon. Without a proficiency bonus, and likely with no assisting feats, they are attacking using a modifier of 2-6 points less than normal. If dexterity is not their main stat, then it is much worse as most ranged weapon attack powers use dexterity.
If this "trick" turns into a regular approach, then sure, talk to your group and come up with a house rule. Having characters carry around copper pieces instead of daggers (also 1d4) as a general habit isn't that satisfying!
As a one time thing to get out of a pinch, or do something cool; I favor letting the player roll the dice and be awesome.
What Power was used?
In D&D4, damage is dealt by powers and certain other effects (like falling). The damage is determined by the power used by the coin wielder.
In the simplest case each character can use a ranged basic attack which will do 1-4 damage if it hits. Add in the dex modifier, only weapons with the Heavy thrown property deal strength damage. Improvised weapons have no properties.
However, there is nothing stopping the attacker from using a much better power that they have access to, for example: Death in the Dark! This attack ignores concealment and does 3[W] + blinded + ongoing 5 damage on a hit.
The rules say an improvised ranged weapon has no proficiency bonus, inflicts 1d4 damage, has a range of 5/10, and weighs 1 lb. (PHB, weapon charts).
It's a freaking gold piece, not a shuriken. While I wouldn't want to take one in the skull, a coin is not gonna kill anyone.
If you tell the players that it has no (+0) weapon proficiency bonus and causes exactly 1 point of damage, no one will ever argue with you.
Hey, it'll still kill a minion.
While Adam's answer sounds right to me, I do know that a thrown US penny, at 2.5g and thrown correctly, can draw blood. (I've done so.) And I like slightly more detail.
The 4e coin is 1/50th of a pound (presumably avoirdupois). These compare in weight to the Sacagawea Dollar coins of the US. The coins (at 8.1g) are 56 to the avoirdupois pound (453.59237g)... That is 3.1 times that of a penny. A good arm can get one going pretty fast.
A penny is pretty weak... the 4e coin is fairly massive. I'd say
While no proficiency is present, I'd suggest one could easily be added. Allow a bonus to hit and increase damage to
To hit should be Dex based.
I have a rogue/bard in one of my games (DND 3.5) that tries this on a fair number of occasions with random objects like coins, stones, mugs, etc. It is part of his character and was for a prestige class which is why I allowed it.
My ruling was: To hit: BAB + Str/Dex (whichever is higher), Damage: 1d3 + Str (for anything fine[coin], diminutive[mug], tiny[tankerd]) Damage: 1d6 + Str (for anything small[stool]). Anything bigger becomes an improvised weapon and I have house rules for those.
These won't kill anyone, commoner or not (DM ruling) but does get the bard flattened/thrown out by the bouncer of the tavern on many occasions.
This was the ruling until they took Master Thrower prestige class (Complete Warrior pg 58), then everything became just a little more dangerous.
Hope this helped answer your question.
All characters have access to a "Ranged Basic Attack" Which does [1W]+Dex.
A thrown coin, without other modifications, uses the character's Dex mod + half level to hit, and does 1d4+dex damage as it counts as a light thrown weapon. The coin has no weapon keywords (again, excepting the belt of the brawler) but can count as a improvised ranged or improvised small melee weapon without trouble.
One concern is that "Any Ranged" the entry for improvised ranged weapons, has a weight of 1 pound. At the same time, the example given is "anything from a rock to a chair" which suggests the weight is more of a placeholder.
Coins as thrown weapons nominally count for improvised weapons. However, there are a number of classes which inform the use of improvised weapons.
One feat, Improvised Missle, allows a barbarian to treat improvised melee weapons as heavy weapons:
In a sense, this applies more to a sack of coins than an actual coin, but the coin itself can be treated as an improvised melee weapon for these purposes, especially if sharpened against the edge of a kerb.
Wearing the Belt of the Brawler would treat the coin as a club, +2 proficiency and 1d6 damage:
Combining the feat and the belt could be an excellent adapation of the idea of the Everyman Hero from Feng Shui.
Another interesting twist is to employ the coin with the Seeker class, Spritbond class feature. In this, the powers would treat the coin as a weapon for purposes of the class powers.
As it is a ranged weapon, the attack and damage rolls would be informed by class powers as normal, and this could be an excellent venue for the above magic item/feat combination. The idea of a character built around improvised weapons like coins, or a gambler character would make for a fascinating and flavourful character.
The use of Ki Foci will pass their enchantment bonus to the coin's attack and damage, making them effective weapons for assassins and monks as well.
The maximum possible damage from a thrown coin, with the earliest level possible, would be from a half-orc rattling ruffian rogue level 2, multi-classed barbarian. [There is some difficulty here with then getting access to the ki-focus implement, but that can be resolved at later levels]
This rogue, with disheartening strike and a coin, the belt of the brawler, barbaian multi-class and the hurl weapon feat:
Attack:dex+2 to hit versus AC
With combat advantage hits at 1d6+2d6+dex mod + str mod, and inflicts an additional -2 to hit against the target of the attack.
Alternatively, you could (depending on how much verisimilitude you want in your game) invoke the Rule of Cool, and say that they're trained in coin-throwing. Unless it's just an improvised weapon. But honestly, a guy who's trained to learn how to throw coins lethally...that's pretty awesome in my book.
It usually shouldn't cause damage. I'd go with something like d6-4, discard negatives and then halve, rounding down: then you'd need a damage bonus of +5 to be sure of causing damage. It should otherwise behave as an improvised weapon.
Enough bludgeon damage should always draw blood (sorry Tilpin).