Bit of an introductory story: I got a discount from my phone company for retention, by threatening to cancel. My neighbour also threatened to cancel after hearing about my discount, but didn't get an offer. Why? He wasn't serious about leaving, and they caught on. I was. If someone knows you won't pull the trigger, they do not have a reason to change what they're doing.
You're Afraid To Kill Them
You can give them options, escape routes, and hints all you want. In the end, they feel invincible. They will continue to feel invincible so long as you're afraid to pull the trigger on killing them.
Apparently I can't kill them, it would cause the whole party to go
This, right here, is why they're right. They have no reason to change what they're doing, because there is no real threat. You're afraid to kill them. They probably know that, on some level. Players are smart and perceptive, even if they only feel things on a subconscious level sometimes.
You have to get over your fear.
Kill Them - Fairly
So, you need to be willing to kill them. But you also need to play fairly. Don't just throw a god mode death NPC at them to wipe them out. Play the game as normal, only if they do something reckless and throw themselves at a challenge they can't beat, be ruthless. If the survivors try to negotiate or retreat, let them.
You have to instill some fear without the players believing you're just punishing them for their play style. Maybe they really can break down the door and fight the next battle without having to be worried. If they're having fun and everyone is playing fairly, that's great!
It's only bad if they charge headlong into situations that should kill them, and you start changing encounters or fudging rolls to save them. Don't do that. Be ruthless, but fair. I know you haven't actually fudged anything yet, but there is a tendency to change targets when someone is knocked down. Don't. Go for that killing blow. Like I said, be ruthless.
It's also possible the difficulty of your encounters isn't high enough, if they can get through them with only the one close call you mentioned.
If you do intend to ramp up the difficulty in an effort to make combat more lethal, it would be worth talking with them before one of your sessions so they're aware of that. It can be a nasty surprise if the game has been played a certain way and suddenly you change the tone to a more lethal one without any warning.
Maybe They Like That Kind Of Game
I know someone who I don't play with anymore, who plays with this style. He absolutely doesn't do retreat, or negotiation. He charges into everything. When he rages, he tries to grab more enemies before his rage runs out. It gets himself and people in the party killed.
It's a valid play style. It's one I loathe. Neither of us are wrong in how we want to play the game, but we're not compatible. He and I simply can't play together, because we want very, very different styles of game. Outside of D&D, we get along pretty well.
The often referenced Same Page Tool can be of some use here.
If you have that situation, you will not be able to change how those four want to play, no matter what you do. The person I mentioned has died like fifteen times and gone through ten characters, while others are on their first. He doesn't care. He wants to smash things in the face, period.
If you have that, you'll have to try to mix things up to cater to everyone as you go along. Maybe there's something that can only be talked past. Maybe stealth is necessary. Maybe they really should run in and stab everything in sight. Mix it up, and try to give everyone something that they like doing.
You won't really know if this is true until you actually do kill some people, and see how they react to it. If they get more cautious, then you're fine. If they just do the exact the same thing over and over again, then you'll have to adapt because that's what they want to do.
plus I hate the idea of paying a cleric and poof the character
auto-magically is among the living once again
This is another issue when it comes to killing people. If you're willing to the kill the party, there are one of two ways of dealing with the consequences:
- Make New Characters
I heavily favor option #1, because my campaigns tend to involve long running storylines and those don't work when the entire party cycles through a bunch of characters. Other people heavily favor #2, because they want combat to be meaningful and death to be a big deal.
The simplest way to make resurrection hurt without getting rid of it is to raise the difficulty of getting it. In 3.5, a Raise Dead cost at least 5000g and required you to have the body intact. For new characters, that was an impossible sum. Even for mid level characters who could afford it, that's a significant amount of wealth, and players don't want to lose tons of wealth (aka: gear, aka: power) by dying constantly. (And if the body is somewhere you can't recover it, then it gets very expensive.)
I don't know what resurrection costs are in 4e, but you can make it a bigger deal to get one via house rules if you don't like how the core game handles it. Maybe it's expensive. Maybe only one person in the country can do it (and she is obviously busy). Maybe a God will demand you undertake a quest for him after he allows your spirit to return.
How hard it is to come back is something you should figure out though, as when you do start killing people you'll want to have this sorted out already. Players should also be told what the rules around it are, if you're not using the standard ones.