read this
http://auto.howstuffworks.com/four-wheel-drive.htm
as far as 4wd on dry pavement it is a big no-no. when the in 4-hi there is tension in the front to rear drivetrain this is becuase the front pulls and the rear pushes. generally, the gear sets or off a tiny bit like a 4.11 rearend with a 4.10 front end, when you are on dry pavement there is no "slippage" allowed and binding is the result.
as far as driving in 4-hi it is mainly common sense, a lot of unexperienced drivers get a false sense of security when in 4wd and drive faster than the conditions allow, the result is an accident. I wouldn't recommend driving in 4wd on pavement unless the conditions deemed necessary. this being how are other vehicles driving, if they're all over the road slipping and sliding then 4wd would be in demand but slowing down is also a factor that would be needed. just remember there are a lot of idiot drivers out there and you need to think rationally because they won't.