Approaching 4 months huh?
At around 4 months, maybe late in the third month, most lo's tend to go on a sleep strike or something. Naps shorten to around 45 mins, often night sleep is affected as a result. What's happening is day sleep rhythms are developing. This is perfectly normal, and will pass in about 3-4 weeks. But until then, you just have to kind of ride it out, start to push toward 2 hours of waking time, but do it slowly since technically they're not up to 2 hours until closer to the 5th month. It's not an easy transition for some, and for others it's a walk in the park.
So, keep your chin up, I promise, this will pass, although you'll want to tear your hair out for awhile. When mine finally got through it I wanted to have a formal celebration!!
*HUGS*
Sonya =P