How fast can one drive in 4 Hi? As fast as conditions allow.
What need does one have to drive highway speeds in 4 Hi? We drive at highway speeds in 4 Hi all the time and have done so safely on interstate trips in our Grand Cherokee. Of course, the Grand Cherokee has
Full Time 4wd.
There are two types of 4 wheel drive. First, is Part Time Four Wheel Drive. Driving on dry pavement (or other smooth high traction surface) isn't recommended by the factory. That's because the transfer case locks the front & rear driveshaft together to turn at the same RPM. This can lead to driveline windup because the front and rear axles don't always turn at the same RPM.
The same isn't true for the tires if the Jeep is equipped with open gears. Open gears let the left and right tires of the axle turn at their own RPM, biasing torque to the wheel with the higher speed. However, that changes if you lock the axle shafts to each other.
The second is Full Time Four Wheel Drive. Full Time Four Wheel Drive has a T case that allows the front & rear to turn at their own RPMs. I'm not sure what mechanism is used to allow the differential in RPM, But I do know from experience that in Full Time 4WD isn't as effective in loose traction.
Although All Wheel Drive drives uses a differentiating T case and is the same thing as Full Time 4WD, the 4WD T case has a low range. All Wheel drive doesn't.
Our Grand Cherokee T case has Full Time 4WD, Neutral and 4 LO (which I believe is part time). The GC also has traction control. Highway speeds in 4 Hi is normal.
In regards to the question
@Claymore has- I think he means "How fast can I drive safely in Part Time 4WD?" The answer is, no faster than driver's skill & experience and conditions allow. What those limits are can only be carefully determined by the driver. In my experience, driveline windup is less severe in a rig with a longer wheelbase. But again, that's something you have to determine for yourself.
In my Cherokee, I've driven at highway speeds in Part Time 4WD on highways in the rain, ice and snow with patches of dry pavement in between. Speeds were, of course, limited by prevailing conditions. I've also driven at less prudent speeds on smooth fire roads. For years, my daily driver was an XJ with an ARB in the front and a Detroit in the rear.
I paid the cost in increased driveline wear, but the extra control Part Time 4WD afforded on the street in adverse and varying conditions has been worth it.