Sleep often goes crazy around 3-4 months. Personally, I attribute it to the end of the "fourth trimester" (Dr. Harvey Karp, Happiest Baby on the Block). Also see Healthy Sleep Habits, Happy Child. For both of my sons (2.5 and 6 months), once they got to be that age, no matter what I did, trying to help them to sleep by rocking, etc. backfired on me.
I settled for not feeding to sleep, getting them on a daily routine, putting them down for naps within 1-2 hours of waking, a nap routine (reading, shutting curtains, singing), a dark room, and some crying, checking on them every few minutes. I think checking on them is VERY important, since one time my son cried for about 2 minutes and then started to sound funny. I went in, picked him up (it was dark), and heard a snapping sound. I didn't know what it was, but he sounded like he was in pain, so I felt him all over, and his ring finger had been caught in a loose thread from the bumper. The snapping sound was it breaking as I picked him up. It was tied off about a quarter of the way down. SOO scary. I don't believe in leaving them to cry without checking on them.
Many people don't want to let babies cry at all, but I think that teaching someone to fall asleep alone (since that is what you want, ultimately) is one of the best things you can do for them. It has been shown to last throughout life. Sleeping is a learned process. Good luck!