What is the highest speed you've gone when in 4-High mode? What gear were you in (ie 4th, 5th, Drive) and what type of surface were you on (ie wet/dry gravel, snow)?
Myself I haven't gotten above 60km/h in 4th with 4H engaged, on snow/slush covered highways (80km/h limit). I would probably be a lot more confident if I had a set of good snow tires, but the tires I have now suck in the slush, that's why I haven't gone faster yet. I had no problems going faster on the same surface in my old Dodge minivan because of that. I can't wait until I get a good set for next winter!