Lol, I remember my son's first 2 years doing AYSO- a bunch of little kids just sort of roaming around the field in a pack, sometimes all chasing the ball at once - in the wrong direction, lol.
This is totally common. At this age, remember, your son is TRYING THINGS OUT. Unfortunately, some of those things cost money, but if you don't allow him to try it, he will never find out if he has any interest at all.
Four year olds have a short attention span, even when the spirit is willing. They get tired and bored fast- this is totally common IMO. I do see your point though about finishing what he starts and I don't think he is too young to start to learn that.
I would say tell him he only has 4 more games and he has to go to them and then soccer is done. But don't expect a lot of enthusiasm about it :)