well you're doing a comparison with a 14 yo. and i'm happy that you brought up that it is a lot different from being 9, but you don't really much info on the 14 yo. does he not compete in tournies? if he wanted to, could he be ranked? there's a good amount of players out there that can be ranked but aren't because of money issues, interest issues, etc. so if the 14 yo is ranked and competes, not obliterates, legit 4.0s doesn't that mean a 9 yo would struggle with 4.0s as well?
edit: as for the inconsistent power players. yes, some are inconsistent. but usually this is due to mentality. they just choose the wrong shots at the wrong times. i have a friend that 17 yo, probably 3.5-4.0. but he is 6ft4'' and his safety 2nd serve is to topspin a ball that kicks about 6.5 ft when it reaches me. he just doesn't know how to take advantage of his serve, he's been trying to s&v, which works wonders so it boggles my mind why he doesn't do it every time. his 1st serves he just goes for aces which is part of the mental issue. imo the ranked 1 9 yo will struggle against my friend because my friend will most likely hold all of his serves, has incredible reach, and is quite fit from track.