Your son has just figured out that he's a social being. And he doesn't know that the middle of the night is not a sociable time.
Firstly, if he sleeps in the same room as you, you need to move him - either temporarily or permanently - to a room that he is alone in.
Secondly, you need to but your son straight back to sleep. If he doesn't need to be fed, then he needs to go back to sleep. Its all part of him learning to sleep through the night. (and of course, that night time is not sociable time). I suggest going into his room, don't pick him up. Say some soothing words, give him a pat and so on. At 4 months, he is too young to be using any kind of crying technique on, so don't try that - it only results in you getting upset and him being completely inconsolable when you give up. He's going to be terribly cranky at you, basically until he learns to put himself back to sleep but you need to persevere otherwise you'll end up being shattered yourself.
Don't pick him up and don't engage with him in anyway - not even eye contact. If you don't nurse your baby to sleep (which, if you do, you might have to stop because it might be part of the problem) then use the strategies that you normally use to get him to go to sleep again.
good luck and let us know how you go.
Hannah (3 yrs) and Harry's (born 22/7) Mum! QLD