We built a simple prototype to test a four-motor Mecanum drive using the VEX 4″ wheels. We used Andymark Neverest 20 motors (about 300 rpm maximum), a frame made with Tetrix Max parts, and a baseplate cut from the sheets of aluminum-clad plastic that were part of the 2015-16 Res-Q challenge.
One problem we faced was adapting the VEX wheels to a Tetrix motor hub. Using some spare 1.5″ diameter delrin rod from our scrap bin, we machined some simple adapters using a drill press.
Here is a gallery of images showing the assembly steps: