What Are the Differences? Prusa MK3.5S vs MK3.9S vs MK4S

The MK3.5S kit is the most cost-effective option to upgrade your MK3S/+ with plenty of new and useful features, including doubled printing speed thanks to Input Shaper and Pressure Advance support.
https://www.prusa3d.com/product/original-prusa-i3-mk3s-to-mk3-5-upgrade-kit-2/

MK3.9S contains everything from the MK4S except for the X/Y/Z motors (Nextruder motor is included).
https://www.prusa3d.com/product/original-prusa-i3-mk3-s-to-mk3-9-upgrade-kit/

The full upgrade contains the MK4S’s new X/Y precise motors (400 steps, 0.9°per step).
https://www.prusa3d.com/product/original-prusa-i3-mk3-s-to-mk4-upgrade-kit/