You pay extra for 4WD... so USE it if the road isn't warm and dry!
The only issue to not using 4WD on good roads is steering.. IF your tires can't slip, then there will be binding... and you will feel it when you steer... specially when you park, even on dirt. I will take it out of 4WD to make a tight turn when I am not moving very fast, like parallel parking because the steering wheel (and tires) will be difficult to turn.
The difference between AWD (full time 4WD) and our Tacoma 4WD (part time 4WD) is a center differential in the AWDs that allows the front and rear drive shafts to rotate at different speeds (as what happens when turning). A 4WD has the front and rear locked together without any middle differential... so you need to be on soft or wet roads so the tires can slip as they roll at different rates when steering.
In a nutshell, you could physically drive 70-80-90 mph in 4WD (High)... as long as the road allows slipping in a turn, or you don't turn. The instructions are for 100 kph (62 mph) +/- limit to engage the shift-on-the-fly 2WD to 4WD-HI.
Seriously, I don't think you need to be going into 4WD at those speeds... but if so, just go in at a lower speed first. Your fuel mileage will go way down when using 4WD, so use it but don't waste it!