Are all of their naps that short, or just one? They might take a short nap in the morning, but should take a longer one in the afternoon. My first response to short sleep cycles was to treat it as a hunger problem -- don't 4 months olds have a big growth spurt? Anyway, offer a bottle, then stick to the routine of "sleep, eat, play," only shorten the cycle. Maybe let them play for half an hour after taking their bottles, then put them back to bed. You don't want to keep them up too long and get them over-stimulated. Remember to do the same thing with both babies.
If they're not hungry, just stay out of the room! Put them in their beds (YAY for you for working that out!) and leave them there a full hour, maybe even an hour and a half (unless their REALLY crying!). Babies don't sleep as soundly as we think they should, and tend to have lighter phases of sleep during each sleep cycle. If you're sitting in there when their bodies naturally start to stir, they will think "Hey! Mommy's here! Let's get up!" if they're all alone they may go back to sleep without any intervention.
If you do feel like you need to just get them up, stick to the "sleep, eat, play, sleep, eat, play" thing.