Nope. Being serious. I think wrestling is a great martial art to start young boys in.
-great for strength and coordination
-there are programs all over the place for grade schoolers (tho maybe not as young as 4)
-at some point around 2d-3d grade or so all the boys are gonna be play-wrestling around on the ground anyway, so at least your kid won’t be spending all of his time on his back calling “uncle”
-IMO wrestling will serve a kid better in “schoolyard fights” than popping someone in the nose
-more grade/high schools have wrestling teams than other martial arts, so he’ll have an intro to competitive sports should he wish to participate when that time comes
-wrestling is a fantastic base should he wish to branch off into other martial arts later
-low/no impact
-you don’t have to worry about weeding out “belt mills” and other pseudo-oriental mystical BS
But don’t ask me. I had my kids stickfighting when they were barely out of diapers! 
I have a black belt (2nd Dan) in Tae Kwon Do, which was not easy to get. The youngest in my school had one at 11, he started training around 4. My instructor was a national champion from Korea (so was his brother). They started learning at 4. I know my instructors kids also started learning pretty early, I believe 4 as well (might’ve been 5). My school is part of the WTF (which, imo, is the better of the organizations, but they’re all pretty similar). The other popular one I see is ITF. If the school you’re looking into is one of these organizations (however, I personally don’t like the ATA because it seems entirely based on calisthenics and seems more like karate, which i don’t like because it seems more sedentary), it should be fine.
What are you trying to teach your child? To defend himself? Pretty much any martial art will fit that bill, since self-defense should teach the child to keep distance and allow a way to seek safety. Athleticism? Personally, I always thought soccer was pretty good. I would add basketball once the kid learns more muscle control and got used to running around for lengthy periods of time (which is what child soccer seems to be).
I don’t know if all TKD schools are like this, but WTF schools in particularly seem to be more focused on discipline. I think martial arts in general, for children, are great for discipline. If you want them to look cool, I was always partial to Wing Chun.