My son is 5yrs old. After 4weeks hydrocele surgery left testicules looks bigger than left a little bit. Is this a sign that his hydrocele is reccured???

Hydrocele. Not necessarily. If the change is progressive in nature then I would report it to your pediatrician. Keep all post-op follow-up appointments as recommended.
No. There is more likely still some swelling from the surgery and perhaps the affected testicle is a little bit bigger than the other. Once things settle down, I am sure they both will be equivalent. Give a call to confirm with your surgeon.