Version 4 is a smaller more reliable transmission that i designed for vex. Version 3 is basicaly the same thing but it is larger and has 2 drive motors instead of 1. Version 4 NEVER jams, and can switch gears on the fly but not stationary.
The gear ratios are 2:1 and 1:2, depending on wheel size this is extremely fast and medium, or fast and slow, or medium and very slow speed.