Yes, I do TKD (ITF style), have done for about 6 years. Now a black belt so in a reasonable position to comment.
4 is fine to start at TKD as long as there is a dedicated children’s class. My association does something called little PUMA’s which is about 4-6, then juniors then adults. They learn things such as blocks and recognising danger (in particular I like stranger danger) and the use of their voice to get out of threatening situations. All done in a playful, enjoyable environment. All of the youngsters at the clubs I’ve trained at seem to be having a great time.
As they get older, they’ll progress into more traditional martial arts, teaching them controlled ability (i.e. using at the right time, but doing your absolute best to avoid) and a lot of discipline. Fitness and flexibility are good and if you’re really passionate, your boy will be able to kick himself in his ear quite easily!
My girl will be starting when she’s old enough without a doubt.