This is a great accomplishment. I myself have gone through similar stuff in the past 6-9 months i've been clean from illicit drugs (i'm on a small dose of suboxone and could in theory raid a cabinet in my house for vicodin, so i know what you've went through). Just dont become content being in this position. Its one thing to have bags in your hand a few times and say no, its quite another thing to 'regularly be in this position' and say no every time.
My point is, dont make it a habit (again). Yes you CAN say no now, but: will you say no every time out of a 100 times? You know that the best way to avoid them is to prevent the situation from happening to begin with, i'm sure that was part of how you stayed clean the past 4 months.
Keep doing what youre doing; stay confident, but don't become OVERCONFIDENT thinking "oh i can see dope all the time, i can hang with these people again!" or "Oh i can start SLINGING now because I can say no!" Avoid the situations entirely as much as humanly possible just like you have been. That vicodin i mentioned earlier? I told my family i knew where it was and to get rid of it. That way the temptation isn't there every single day; because i know if i have a 'bad day' its entirely possible i'd cave. I have 0 intentions to, and i've had bad days and not, but the right circumstances its a definite possibility for me personally.
Don't get it twisted I'm not saying you were planning on doing either of these things, nor accusing you of thinking it. I'm just putting the info out there for your benefit. An ounce of prevention is worth a pound of cure, ya know?
And once again: good for you! It's not the easiest thing in the world to do the right thing, but you did it.