No, it is not safe to drive a 91 4Runner in H4 gear on the highway. H4 is a 4-wheel drive high range setting that is meant to be used when you need more traction off-road. It locks the front and rear axles together and provides 50% of the power to each axle. This can cause your vehicle to become unstable at highway speeds, especially if the road is wet or slippery.
H4 gear should only be used when driving off-road at low speeds. When you are finished off-roading, be sure to shift your vehicle back into 2WD or H2 (4WD high range).
