i believe the little paper insert on visor over drivers seat says 50mph/80kmh is limit in Hi-4 for automatic disconnecting differential (ADD).. was reading it today at SONIC while waiting on lunch.
honestly though...I do not think any of my other previous vehicles ever had a limit as most said you could switch into Hi4 at any safe driving speed. Not sure why it would make a difference...other than a Liability Lawyer input to keep someone from thinking they are in Baja500 and losing control when they go airborne and then land with wheels turned and wonder why their vehicle is all over the place...LOL