i think people refer to that as "the 4 month wall."
lots (most, maybe) have a sleep regression around 4 months. it could be that your baby just needs more mama.
things you could try (i know pp mentioned some of these too)
swaddling
white noise
swinging (you cold try the amby baby motion bed)
staying close by for 20 minutes after he's asleep to help him stay that way
change his sleeping surface (soft, hard, scratchy, warm, cool)
make sure he's in 100% cotton from head to toe
a recording of mama and daddy softly talking playing in the background
these are all things we tried. not to discourage you, but my daughter didn't sleep for more than 45-60 minutes until she was 18 months old. i think she's pretty far at the high-needs extreme, but she doesn't have any problems. that was just how she was.
you could get The Baby Sleep Book, by DR. Sears. it helped me see what was normal for babies, and it gave us quite a few ideas on how to parent our little girl at nighttime.
good luck mama.
s