Help!

My 4-month old son has been waking up about 4 times a night for the last few weeks now. He'll go to sleep around 8/8:30pm and sleep for about 2 hours, after that he wakes up every hour / hour-and-a-half. It's making me lose way too much sleep, but I don't know what to do about it!

He'll usually go back to sleep as soon as I nurse him, within minutes (sometimes even less than a minute!) so I'm pretty sure he's not hungry. I don't believe in letting him cry himself to sleep and besides, that would take so much longer! He does the same thing during the day he only take 10-15 min naps and wakes up.

What do I do to make him sleep longer/wake up less often???

Could be teething. Could be anything. Babies wake for all sorts of reasons, and just when you think you have a pattern going, something changes. If you don't believe in CIO (I don't either -- I mean sure I believe it exists, but all the research I've read says it's harmful neurologically and emotionally), what about co-sleeping? My DS slept 6-8 hours in a row from about two weeks old, then suddenly at six months, he started doing just what yours is. So I pulled the side off of his crib and strapped it to my bed so I could roll over and nurse him back to sleep with minimal effort. It still sucks, but it's way better than getting up to soothe him. And co-sleeping actually causes your sleep cycles to align more closely with his, so you feel more rested.
The good news is, it's temporary. DS still wakes, but he's going longer stretches, and I know I'm not going to be nursing a six-year-old to sleep.
I've heard recommendations for Elizabeth Pantley's book, The No Cry Sleep Solution, but I haven't read it yet myself.