I wouldn't worry about his weight at 4 months. He's still a baby and will be running all over the place. How much do you feed him? I think kittens of that age should eat as much as they want but by the time he gets to about 6/7 months (esp after neutering) you might want to cut back a little if you think he's not growing out of it, especially if you feed dry food. But I wouldn't worry too much at the moment. Kittens undergo various growth spurts and in a couple of months you might find he's suddenly gone long and lanky!