[RPG] How to handle someone who never ever wants to pull

Player 4 sounds like a pretty novice/young/both player. I'd try one or more of these:

1. Explaining issue - I'd try to explain the player that there is a difference between kill-everything-that-moves Diablo and your average-rpg-with-some-battles-and-story. It might be that they are not familiar with how to play a table-top RPG, which generally is more about social interaction than mindless bloodshed and endless dice throwing.

2. Solving it in-game - It is almost always possible to make someone or something not kill your player. In this particular situation, the five people could just incapacitate your player, throw him into jail and forget him. Force him into slavery. You can try to find your way around this stupid action and use it as a plot hook, while making it uncomfortable enough for the player/character that they will know they did something wrong.

3. Changing your vision - Heck, perhaps the players don't want your fancy RPG-ing stuffeyz things. Maybe they want you to throw them into a randomly generated dungeon with randomly generated encounters from the book? Their expectations might be quite different from yours.

4. Wait a couple of years - At worst, the player might just not be capable of an organized and sane gameplay due to simple stupidity or immaturity. Maybe they won't learn their lesson even if you bribe them with money to play coherently. You have to declare a line after which you will tell the player "Play like a normal person or leave". There is no point wasting anyone's time with someone who just wants to jackass around.

5. Warn and talk about it - (added after seeing the clarification in the comment) If usually she's a normal player, then hm... Yea, as others have suggested before, first of all I'd make sure she is really aware of what consequences her actions can have. Asking her if she is Really Absolutely Totally Whoever-forbid Sure at first, and if she insists, you might try going the...

6. Your character wouldn't do that - I have problems believing a raving lunatic or some war fanatic would do something like this character. If all reason fails, unless characters have some serious problems with themselves they would never do something as stupid.

But all the above advice aside, the best you can do, in my opinion, is to just talk with such players and inquire why they did that. This should give you the necessary insight to resolve the problem.

Well. There is much advice and hints and tips and tricks to be learned. But I will keep on task, and I will help you in your specific situation. You can ask more questions later, relating to any other aspect of running a game that you want.

The Journey

The start of the ambush begins as the players first start on their way. You describe their surroundings, what kind of path, or road they are traveling, you ask them to describe how they are traveling, what things they bring with them, etc... because obviously as the GM you should know what your players are up to, how they are dressed, how they prepare themselves, etc. This lets the players describe to you their preparedness without them knowing they are doing that in preparation for an ambush.

Example: The players are just packing up their camp on their second morning on the road between two towns. You describe the path to them as muddy from the nights rains, and the scattered trees as glistening with rain drops, the breeze is sharp and slightly cold, and the sun is still rising over the horizon, You ask them to describe how they dress, how they prepare, because after all - the journey awaits. The warrior describes to you how he is wearing his armor and cape, but that he puts his bow in the wagon because he fears it getting dirty on the mud if he walks with it, and the wizard explains how since his robes are now soaking wet, he switches to his traveling gear (which incidentally, don't have his secret pocket full of alchemist fire flasks). etc etc.....

Misdirection

Here is an important part of the ambush, you want to check if maybe your players characters can detect the ambush, but you must do it in a way that they do not feel they are about to walk into an ambush. You want them to roll their perception (spot, listen, smelling, 6th sense - whatever) without knowing its for an ambush, and also after the roll, they should be feeling safe and unsuspecting.

Example: You describe to the players how they spot the carcass of a dead deer on the path ahead, and ask them to roll their perception. You generously describe the deer and its surroundings to those with higher rolls, and mention that it poses no threat since they can clearly see the animal was killed by a hunter who took its antlers. The players who may have initially been concerned with a dead body, are now calm and unafraid. And you have their perception rolls for the scene.

The Twitch

Something is not quite right. Some of the players may start to be suspicious, this indicates that the players have caught on to the fact that something may be afoot. Preferably this happens after you have already got everything in place, if not - you need to act fast before the ambush is too expected. Regardless, this is the point that you will be at right before The Reveal.

Example: A player may ask you "Will anything interesting happen here? Why are you describing to us this boring forest as we walk by?" Or "Dude, this is totally an ambush!" (okay, I don't have amazing examples)

The Reveal

This is the moment you have been building towards, it needs to be grand and really make your players feel surprised. Regardless, once the ambush is revealed, your deception is pretty much over, and combat (or fleeing) will begin.

Example: Maybe an arrow suddenly flies past them, narrowly missing them, signifying the start of the ambush. Or perhaps the shouts and screams of a dozen Kobolds suddenly surround them from all sides as they charge forth from the forests concealment.

On The Table

About what you do physically: Only pull out the map after the ambush has been revealed. Until then this is just a normal day traveling the road, and you describing the wonderful scenery. Based on their perception rolls and how they describe their characters, you will judge their preparedness (including their initiative rolls) to see if they can draw weapons, or prepare anything else for the ambush.