C.,
I thought I would respond because I had the exact same thing happen to me. Right before my son turned 4 months, he started waking up every few hours (prior to this he had been basically sleeping very well since about 6 weeks. After his 4 mo. check up we started giving him rice cereal with the same thought as you, maybe he will sleep longer. We also had a different problem of him turning over onto his belly but he had not mastered the art of turning back over belly to back...so we would have to go flip him several times. He soon learned how to flip over, but was still waking up. We too were getting frustrated. I had read and somewhat followed the baby whisperer. My friend told me about the baby whisperer answers all of your questions...and in there, she talks about a 4mo old should be on a 4 hour feeding routine (we were still doing 3 hours) so we switched to that...it seemed to help somewhat. But then one day, he went back to sleeping through the night???? So who knows...was it the routine, was it a growth spurt or bout with teething?? Looking back now, I think it was just several changes all at once. My husband and I had both been on a long Christmas/winter break and we were back to our work schedule, etc...and my son was going to my mom's for "daycare." I think all of these changes could have been the culprit, or it was just normal "changes" he was going through at that time.
Maybe with the "changes" you have had in your family (you being in the hospital, your hubby doing things differently than if you were there (not good or bad, just different), and your son getting older and reaching more milestones, could be the reason. Also, if he is more active and starting to do new things, I heard that can play a role in their changing sleep habits. I say stick with it, be consistent (of course make sure nothing is wrong with him medically)and hopefully this will be just another "phase" in the many they will go through. Now, having said all of this, I am first time mom and you are more experienced, but I thought I would just share what I went thorugh, sounded similiar to your situation. If you are up for reading, the baby whisperer is a good resource (the baby whisperer answers all your questions, seems to have more info. geared toward older (beyond newborn). Good luck!!!