Re: 4 months and major spitup
A close friend of mine experienced the same thing with her son. He spit up so much after every nursing session that she carried what looked like a suitcase-size diaper bag with changes of clothes for her, the baby, and a million burp cloths. Her son, like yours was a happy baby, gained weight well, had plenty of wet/dirty diapers, and had no other issues. She tried making sure he stayed elevated for 30 minutes or so after each nursing session, but, really, I don't think anything she did actually helped. She just learned to be prepared for the spit-up (and the inevitable comments from everyone around that there must be something wrong). At about 7 months, it started decreasing significantly and completely stopped a few months later.
Mom to DD1 Peyton (5/12/06), DD2 Micah (4/1/09) & DS1 Ryland (10/27/10)