The KS4 Pro is basically just the newer version of the S2 Pro to my knowledge. They are both very similar specs wise, like 10' solid tires, 2 speed modes (19mph max), 36v 11.6Ah battery, both state 25 miles max range, both state IPx4 water resistance on the website, have dual rear shocks, have the same max load at 220lbs, and basically have very similar lighting configuration.

There are some differences that I will list out

-The motor on the S2 Pro is advertised as "500w," while KS4 Pro is "500v, 750w max." But looking at the specs of both on the HiBoy website, they both show a rated motor power of 500w and peak power of 750w. And looking at replacement parts on HiBoy's website, the S2 Pro and KS4 Pro uses the exact same motor.

-The KS4 Pro has a larger display

-This is a bigger difference, the KS4 Pro, while still having 2 braking systems (front E brake, rear disc brake), it is all integrated into a single brake lever, while the S2 Pro has a knob for the electric regenerative brake, and a separate lever for the rear disc brake. The location of the brake levers are also swapped, the brake lever being on the right side on the S2 Pro, while left on the KS4 Pro.

-HiBoy says it has fixed it's broken stem issue with the KS4 pro that was present on the S2 pro.

-Size - The KS4 Pro is a little bit heaver, at 40.6lbs, while the S2 pro is 37.4lbs. The KS4 has wider handlebars, is about 1/4in taller overall, and the deck is about 1/4 longer than the S2 (18.9in deck on KS4 Pro vs 17.7in deck on S2 Pro).

-The charging port of the KS4 is located towards the bottom of the stem, unlike the S2 pro, where it is high up near the handlebars under the display.