First, any alpha leader crap... forget about it. Your puppy is just that... a PUPPY!! He does NOT think he is your "leader". He is testing his limits and learning what is right and what is wrong, much like an infant child. By using dominance, you are going to get an even more aggressive pup. I have seen dogs who these techniques were used on who only got worse, and had to be fixed by a different method.
Secondly, I highly doubt your 4 mo. old puppy was truly being aggressive. It's possible, but unlikely. I've never really met a young puppy who was honestly being aggressive. Hard to say without seeing what happened... but I doubt it. Jackson "growls" and grumbles a lot during play and training. He is the farthest thing from aggressive. Pups make a lot of noise! I play tug w/ Jackson and he will growl the whole time while pulling.
Also, what's so wrong with being your dogs buddy? I've accomplished a lot with being my dogs friend, his teacher, his protector, more so like a "parent" I would say. Now don't get me wrong, if Jackson is about to eat something he's not supposed to, or is barking too much, I give a "Eh-eh!" or a "no!" but in your case, I think it's just a matter of being a puppy and needing a teacher.
First thing I would do is a teach a solid sit, stay, down, and "leave it".
Some good videos:
What not to do and why: