My 4-month-old is drooling and chewing on his hand. Does this mean he's teething?

It does indeed sound like teething symptoms along with scratching and pulling his ears if he does. He's probably itching on his gums. You could make an ice (or breastmilk) pop for him or put his teething toys into the freezer to help him soothe his gums. Even though he may have teething symptoms now, he may not have his first teeth until several months later.