Get a simple Strep test from your pediatrician.

Yes, he could. The usual signs of strep along with a sore throat would be a fever over 101 and swollen glands in the neck. And in a 4-year-old, also lethargy and irritability. You should have the pediatrician see him and do a strep test, and start antibiotics as this is a bacterial disease and should be treated with medication if present.  

Strep throat is a common cause of severe throat infection in children. It is even more common among daycare/school age children. However, not all sore throats are strep throats, many are caused by viruses, which typically clear without the need for antibiotics.

The only sure way to know if your son has a strep throat is to do a strep test or a throat culture. Viruses, allergies and other bacteria can and will result in sorethroats. Take your son to his pediatrician and ask him to do a strep test on him and if positive he will be able to treat him as necessary.

Throat irritation is a symptom of strep throat, but is not specific to strep throat.

Unlikely. Could be viral, could be allergy, could be reflux, could be acidic food sensitivity, could be surface drying from mouth breathing at sleep.