With the short wheelbase, you'll be spinning 360's on the Interstate before you reach the limit of how fast you can go in 4-high. Those big concrete barriers make the unstoppable stoppable.
Seriously, you can get a lot of confidence in just how well these things can go, but the shorter wheelbase and weight distribution can cause some unexpected things to pop up. I've seen one spin for no apparent reason going down the road.
If you're looking for hard numbers, you shouldn't switch into 4-H from 2-H going over 55 MPH. 4-H is recommended for "as fast as conditions allow".