In 4hi, you can safely go up to the speed limit, about 55 or so. I've driven my Suburban, '04 2500 with 8.1, up to 65 before on the interstate, I didn't realize it was still in Auto mode, but it was fine, even though it made a whine. Some diesel guys run the 1/4 in 4wd, and they get up to 85-100 mph. I wouldn't recommend sustained use of 4wd on the highway, but it can be done. The manuals say you can shift on the fly up to about 45 or so. And I've done that.
As far as 4low, I guess you can go as fast as your rpms will allow. I've gone about 27 mph in mine, taching about 3500-3700 I think, in OD. LOL Again, just testing it out, but I'd limit 4 low to about 10-15 mph.