Congratulations, you've just hit on one of the "surefire ways to get flamed on XOC" topics (how fast can I go in 4hi... queue "slow down a-hole" comments)

The only real limiting factor in 4 low is keeping the RPMs in check... 12mph seems awfully low, though - in the gen 1 it is more like 25 or 30mph (with stock tcase gears).

In 4 high.... I dunno. All the front end stuff is spinning - and it may not be able to handle spinning that fast.... or maybe it isn't balanced that well and may start vibrating at higher speeds.