My daughter did the 30 minute naps too..it was amazing to me that it was on the dot. For her, I think it was a mixture of the sense sensitivity that occurs at 4 months and the fact that she couldn't put herself back to sleep. I believe that she would half wake at 30 minute intervals (like we do in our sleep and we roll over or look at the time or whatever) and not be able to put herself back to sleep. (now at 9 months she wakes at those intervals sometimes and squeals and then goes back to sleep) I think the sensitivity made it even harder for her to go back to sleep. I could tell that she wanted to sleep longer because she would. not. be. happy. when she woke up.
We put in black out blinds in her room, got a sound machine, and turned off her night light.
I am not a believer in the cry out method, but one night after she was already down for the night, she woke up as usual. My husband and I stopped what we were doing and stared at the monitor. I can tell when she is in absolute distress and when she is just complaining. If she were to cry as if in absolute distress, I would have gone in there, but she didn't. She complained for 15 minutes and then went back to sleep. Now, once she is asleep, she can put herself back to sleep. She still cannot put herself to sleep from being awake, but we are one step closer.