Hi A.,
I am a surgical pediatric nurse and I have taken care of many children after their hernia surgeries. I can tell you from my experience as a nurse, that children typically do very well after this surgery. The trick is getting them to take their pain medicine. Since your son is 4, hopefully it won't be to much of a battle. He is old enough to understand that the medicine will help him feel better. Where is he having his surgery? Will they do it laproscopically? If it is done laproscopically, kids recover very quickly--usually even outpatient and go home the same day, sometimes stay one night. It will take a couple days before he's feeling "good", but will have to take it easy for a couple weeks, so he doesn't get another hernia. Also, if its done laproscopically, then he will probably have a lot of gas pain because they use air to fill his abdominal cavity during the surgery. The only thing that helps this pain, is walking around to disperse the air back into the tissues and blood stream.
I hope this helps reassure you! Let me know if you have any more questions. :)
~K.