From what I am reading it doesn't sound like your boss is very good at being your boss but I don't think it was a malicious or intentional breach of trust.
I personally would approach him. However, you need to "step carefully". Because if you approach this the wrong way or are harsh to your boss and just tell him off (which I am sure you might like to do) it could end very badly for you.
Approach him. Do it in a very calm manner. Maybe ask, "Hey can I have a word with you in your office or somewhere in private?" This should definitely not be done at a meeting between all of you or out in the open where the others can hear you. Once you guys are alone, calmly ask "Did you tell ___ about what I said about him the other day when we talked?" If he tells you that he did then reply with calm questions like "What was your reason for telling him?" and "Why did you assure me that you weren't going to tell him what I said?"
Make sure that when you are asking these questions that you never come off as angry or frustrated which I know you are. The trick is in the way you are asking the questions and talking about the situation if you come in and just go off with a question like "WHY DID YOU TELL HIM?! YOU TOLD ME YOU WOULDN'T!" That's not going to be productive for anyone.
There is also the option of not confronting your boss. By doing this you are going to avoid any possible outburst that you may have at the time but also you are just going to keep your anger built up and next time your boss does something that betrays your trust you might feel more hatred and maybe you'll say something you regret.
The best thing to do is to address it. So you can move on. Your boss will know he betrayed your trust and he can learn from that. And everyone can move passed the situation. Because, especially in a small company like you are in, there needs to be good camaraderie between coworkers because those are the only people you work with and there is not really time to be angry at each other. Think about it, if you are angry at your boss then 1/4th of his employees are having problems and maybe you tell your other coworker who is your friend and he sides with you. That's half of your boss's employees that are mad at him and, again, it's not productive.
You need to address the situation; just make you sure tread carefully, calmly, and quietly. Don't make any waves.