Hi,
I have a 9 month old son and until about 2 months ago, his naps were also short. He would take one in the morning, reliably most of the time, for about an hour. But afternoon naps were a bear. Sometimes 30 mins, sometimes 45, sometimes not at all. I tried everything as well and was also frustrated that he'd wake up from the short naps still obviously tired. I think it was just his age -- at 4 - 6 months, they don't always take long naps apparently. We did do the Weissbluth method (leaving him to soothe himself) for naps most of the time, but until he slept through the night consistently, his naps were pretty short (1 hr to 1 hr 15 mins). He started sleeping through the night consistently at 7 months and naps have been getting better and better. 1 hr 30 mins - 1 hr 45 mins at 9 am and 1 hr 30 mins to 2 hours at 2 pm. Having said all of that, there are still occasions when one of the naps is 30 - 40 minutes for some reason. Nothing is different in the prep -- it's just short -- and I've come to the conclusion that sometimes, this is just the way it is. Especially now that he has learned to crawl, he seems more stimulated and gets into the crawling position in the crib and wakes himself up earlier than he otherwise would. So...I guess my advice is not to worry too much. 4 months is pretty young from what I understand to have long naps. They will get better. Stay consistent with your times and preparation and they will fall into place. Hope this long-winded message was somewhat helpful!