As has been stated, there is no real maximum to run in 4x4 high. There can be a maximum speed to engage it, but that's about it.
I would not run down the highway in 4x4 doing 60 mph unless it was an emergency. I used to laugh when I ran my Ranger around in the winter, it was all I had for a couple years and it was always kinda funny to see 4x4 trucks and SUVs in the ditch on the highway.
4x4 will help you get going, but if you don't understand how it works, you can end up in the ditch too, IMHO. I ran the '89 BII Eddie Bauer I had (and sold...stupid) for two winters up in the land of lake effect snow near Erie, PA. I usually left the front hubs locked in and ran around in 2wd until I needed 4x4, then I'd just flip the magic lever and away I went. Once I was headed towards a light and it changed, so I tried the brakes (wasn't going more than about 20 mph), and no matter how lightly you'd touch the brakes, you'd slide. I tried kicking it in 4x4, but that didn't help matters, it was just too slick out. So rather than skate though the intersection I cranked the wheel over and tapped the gas, front tires caught enough to swing around and drive me into the snowbank, which stopped me. Backed out and creeped up to the light. 4x4 go does not always mean 4x4 stop.
HOWEVER (and I hate to say this but it got brought up), 4x4 can help give you more control for braking in certain conditions. I would not trust it for helping to stop you any faster on a highway or whatever. But I have found that when you're moving down a slope SLOWLY, it can improve your ability to brake and steer. It doesn't seem to help all that much going forwards, it's most noticeable going backwards and I believe that the reason is that drum brakes do not grab as good in reverse as they do forwards, so by linking the front and rear axles, your front brakes can now help out more with stopping. I've tested this on our driveway (which I typically back down and it does get rather slick despite clearing the snow as much as possible - it's a limestone driveway).
I have also had experiences on the road where 4x4 low was the way to go.... driving through a nighttime blizzard with gusty winds and rapidly drifting snow. Never went above 25 mph, most of the time was 10 mph. I would not recommend it.