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He's be hurt more if he found out from someone other than you, and the guilt is just going to keep eating at you. Sit him down, tell him what happened.

Love and lust are separate entities. What you gave in to was lust and playfulness.

Alcohol could make one forget the price of living in the moment, and you're young: you're probably new to the realm of commitment and that sort of activity being reserved for your monogamous partner... it was weak, and wrong, but that doesn't mean that you actually DECIDED to betray your boy or decided to do anything at all.

It would be best for you to tell him and for him to forgive you so that you two can keep what you have which is so valuable.

The only problem is... how can he -how can YOU- know that this isn't going to happen again? How can you be with your boy under the terms you've agreed upon without knowing whether or not you can do this? If you haven't worked out what you've needed to in order to have your priorities straight, like all teenaged girls need to go through to find out what they want in a boyfriend... what they need... then what business do you have agreeing to a monogamous relationship or having any claim to being a soulmate or having a soulmate?

You need to be brutally honest with yourself about what you need, what you value, what you have... and you need to be honest about why you are going out to clubs and bars with guys and how you couldn't stop doing what you did. Is something not being satisfied in you? Have you finished experimenting with relationships and deciding what you want in a partner?

If only open relationships or relationships without monogamy are what's best for you, or the only relationships you can handle, then you can't have this boy, and if you can't tell your boy what has happened then your relationship with him is destined for failure.

You should tell him.
But if it's really unlikely that he'll find out (which it doesn't sound like because 2 people other than the 2 of you know about it) and you can get rid of the guilty feelings, don't.

4 months isn't very long in the grand scheme of things and in all honesty, you're likely to lose him.

On the other hand, it's early days, he may still be as infactuated with you as most new couples are with each other. In which case he'll be heartbroken but he'll forgive you.

You didn't sleep with this guy, you drunkenly made out with him.

A tip.
When you do tell him do not use drinking as an excuse. Mention that you were drunk and not feeling yourself but say no more, it'll aggrivate your boyfriend even more.

If he forgives you beware that he's not going to trust you for a long while, or be able to properly let it go for a while and may even bring it up in the future and make you feel like crap all over again.

Ooh, that's tough. =/
I understand where you're coming from, but I also see how your boyfriend could take it the wrong way. I think it's best to tell him before someone else does. I can't garuntee you that he won't leave, but if it is true love, chances are he'll understand. We all make mistakes and do things we regret, especially while we're drunk. But if you honestly have no emotional attachment to this other guy, and you didn't go farther than kissing, that lessens the blow.
There is one simple lesson to learn from this though: drinking in the company of 2 boys you don't know very well is probably a bad idea.

RAWR FIX it pl0c

Ah... bad... Really, u were under the influence, u weren't urself, and any guy that it is possible to be with for the rest of your life should be able to work through sucha problem with you. Love doesn't mean you never have problems. He should be able to see that the act of you cheating on him was not an act of love and that by telling him you still love him. You need to tell him, beg his forgiveness. Make sure he knows you still love him, then sit back and give him a little time to decide what's going on. Being a guy myself, I think this is solid advice.