What must i do when 4 year old has temperature of 38.5 and his eyes are red?

Your actions are up to you. Such low grade fevers are common to many self healing viruses but this may be a prelude to something requiring treatment. The eyes are a common entry point for some viruses but may also be the location of an emerging local infection with drainage. If the kid eats/drinks/plays normally I would just monitor. If this stops consider a visit.

Most likely your child has an infection. If there is any eye pain, discharge from the eyes or findings that are concerning, please see a Pediatrician. Local bacterial conjunctivitis is usually associated with thick eye discharge. If you notice a rash or swelling, please discuss with your Pediatrician. Watch for prolonged fever, peeling skin and changes of the lips or feet/hands. Encourage fluids.