I am not a dad, but 4 does seem a little young. Martial arts is teaching self-control as well as defence, is a 4 year old really able to exert mental control over lifes rich tapestry? I doubt it.
When he is older, may I suggest Judo. I tried various martial arts when I was a nipper and found Judo to be the best for strenght and everyday application. Judo has also been the favoured martial art for our armed services and Police force (either Judo of variation such as ju-jitsu or the home office UDT {unarmed defensive tactics}).
In most situations, say a robbery or a barroom brawl, my money will always be on the judo trained man as opposed to karate, kung-fu, thai boxers et al. Also, there is less risk of civil or criminal proceedings against the judo man as there is no striking involved, only holds, take-downs and locks.
Further, judo is the quiet mans choice. The karate kid (no pun intended) will often show off and leap from school tables pretending to be Bruce Lee, but the judo kid is a reserved, dignified child. Safe in the knowledge that he could actually whoop the ar5s off any kid in school.