Re: 4 year old
I'm close with my nephews and I watch them a lot. I've been showing one of them who's a little older than your son the nuances of the game. He got a new glove the other day and I went out and threw him some low underhand pop-ups and bounced him some grounders.
I made sure to teach him to keep his glove down with a hand over it, like a venus fly trap and showed him how to shuffle his feet.
For as young as he is, he caught on pretty quickly.
My oldest nephew is in second grade and likes to try to take my head off hitting the wiffle ball back at me.
I actually look forward to when they're a little older, as I've always wanted to try my hand at coaching.