SA5 is actualy pretty slow due to being driven by a deltang which has a pretty low voltage limit and using 30:1 motors.
its replacment will be: faster, even lower, and a more powerfull flip.
It would be easy to do, other than it takes like an hour per 'wheel' to assemble and fine tune. the main problem with this idea is that walkers have very little pushing power as theres not one constant serface gripping the floor and i don't think adding 4 'wheel' drive would solve this. Thus not much point in making one like that. I have a few ideas on changes to the mechanisum to make it have more pushing power, as the main donfall to my walkers has always been getting pushed easly. so possibly if that works and it does become good at pushing then a 4wd robot would be a good idea.