Ok, first of all, a bit *HUG*!!! Close your eyes, breath deeply for a few minutes
Congrats, you've just reached the 4 month nap battle! During this time, day sleep develops, and it's developmental and normal for your lo to be taking 30-40 min naps consistantly. It is a stage, and will start to go back to normal in about a month or so, or at least it should. Just keep with what your doing, try to keep him rested, and don't start any props. Trust me, they'll be about as helpful as anything you're doing now. There is a light at the end of the tunnel...but it's hard to see it when you're in the middle of this stage of development. When I went through it I actually ended up starting treatment for PPD, as this was a really rough time for me and I was taking the whole napping thing waaaay to seriously (as in, my mood revolved completely around whether or not my lo napped well).
So, it's ok to cry and get upset, but just remember (even though you really really don't want to) that it is a phase and will pass. Also remember that your lo can sense your mood. So if you're upset, he will be too, and it will affect his sleep habits. If you're relaxed about it, he may be more apt to get through this faster.
Let us know how things go!
*HUGS*
Sonya =P