how do you teach a four year old to play chess?

of course with the good old way. its called stick... 😛

In elementary school, the chess club coach would always give us cheese fish and orange juice to drink. Mmm...Cheap food, but I still think of it as a delicacy nowadays. 

There is nothing better than eating cheese fish, drinking orange juice, and playing chess at the same time. We even tried to soak the cheese fish into cups of orange juice and have them together. Sometimes as a child, you don't have many expectations. All u have to do is to put all of ur senses work, away from boredom and distraction. If u can satisfy these basic needs (for a 4 year old), it won't be difficult teaching them the basics of chess. How to make them try to stay super focused and do deep calculation, is another question.

There isn't much in chess that really attracts kids and toddlers. Perhaps the only thing I can think about is the motivation of winning.

I've seen some people work with really young kids.

If they can make legal moves, that's great.

You can do exercises like you set up a position, then ask "how many squares can the bishop move to?" and they have to count.

But doing more than this I'm not sure, because 4 is pretty young...

Don't bother if he cannot tie his shoelaces. 

Teach them if they're interested. No more or less. That's the only reason I started playing at all. I was eight and half when I did. Regardless, interest matters the most.