So I guess the emphasis is on finding a good instructor rather than looking at each discipline?
If you had the same instructor with the same level of expertise and skill set across all disciplines... what would be the most suitable MA to the style I've described?
Hey Dave, Welcome!
You have received some excellent advice, I'll just add that not only is good instruction is important -- good children's instruction is paramount. There are a lot of instructors out there who may be fine teachers among a teenage or adult clientele, but are not as good with children, nor are they good with teaching kids in a way that is age-appropriate. I've seen MA instructors talk to 4 and 5 year olds in a tone that I wouldn't even use with my team at work -- and my crew is a damn tough lot (former Marine, former police officer, former corrections officer).
Personally I'm not opposed to starting a 4 year old in a quality school...but I really like the idea of starting him in gymnastics first and then enrolling him in martial arts a little later. I think, pound for pound, gymnasts are perhaps the strongest and fittest athletes out there. Its a great pursuit in its own right, and also a great foundation for just about any sport. It will give him a significant advantage in martial arts training.
As far as the most suitable MA...I don't think there is a best choice. I don't think there ever is. In addition to the quality of instruction, there are also going to be factors such as distance, time (scheduling), and cost -- factors that likely influence how long the parents can support such an activity. Find a good place to start and then see if his own interests go in a particular direction...this is the sort of thing you can only learn by experience. If you pick a good instructor to begin with, then he'll get the most out of the training, whether his interest is in that particular style, a different style, or something outside of martial arts all together.
Good luck and keep us posted!