K.S.
Don't beat yourself up about the vaccine. In all likelihood, your baby was already exposed to the virus before the shot. It can take 2-4 days to incubate.
Anyhow...We have never had a confirmed case of rotavirus in our house. But my youngest got slammed by a notavirus and it was terrifying at 8 months; I cannot imagine a tiny 4 month old going thru this.
My advice - DO NOT HESITATE to bring your baby to the ER. Such a tiny body can go into dangerous dehydration very quickly. We had Dylan in and it was scary the amount of fluid they had to pump into him thru an IV. He couldn't make tears, his eyes were sunken and even the dimples in his hands disappeared. We ended up back at the ER 24 hours later for another IV.
Diaper rash - They can and should prescribe a 3-part connconction they literally call Butt Paste.
BTW - Every doctor seems to recommend something different. But ours said no juices (dehyddrating and often bowel loosening). He said as long as we could get him to drink breast milk, formula...That was all he needed. And gosh, if you can get a 4 month old to take yogusrt? That might help.