Yep, it sounds like a 4 month developmental thing. My DS2 started sleeping really badly around that age. It does pass so you can just ride it out if you are both happy to.

If you want to encourage more sleep, I used this trick - I would go into my bub's room a few minutes before his first sleep cycle was about to end (for him it was at 40 minutes, it sounds like your DD's is 20 minutes), wait until he stirred slightly (he would sigh and twitch), then I would pat his tummy gently for about 60 seconds. This usually helped him go through to the next sleep cycle (it used to work for one of his three naps anyway!).

But if your both happy and coping and want to keep doing what you are doing, then go for it! I'm sure she will go back to sleeping longer soon.