you have confused 15 and a half, with hands and fingers,,,
60 inches makes him 15 hands, every inch is a finger,, so in horses, 15.2 doesnt mean 15 point 2 hands, it means 15 hands and 2 fingers, IE 62 inches. So you get 15.1 (61 inches) 15.2 (62 inches) 15.3 (63 inches) and 16 (64 inches)
Saying 15.4 would be like saying I am 5 foot 12 tall.
If you could get a brake controller for an S10, which you probably can't get plug an play, you will have to find someone to wire it in, It still would be iffy, but doable. Your not going to find what you need at Wally world. I might try one of those trailers empty, but loaded,, if you have to hit the brakes in your truck hard you will likely jack knife.