Thanks for your query.
The surgery of hydrocele
is a short procedure around 20 minutes(done in day care) and since he is just 4 yrs the doctor may give him a short general anaesthesia
.Post surgery the surgeon will prescribe for him a pain killer syrup and an antibiotic to prevent infection.Till the dressing will be on(may be 3 days) you should not give him bath and you can just give him a sponge.Later you can give him bath and mop dry the area.
If after surgery he is comfortable and does not have pain he can go to preschool on the doctor advice
Hope you find this information helpful.