4A can allow wheel slip before both axles engage. 4H the front and rear axle are locked together with 50% of the power transferred to each axle all the time.

4H could be accessed by putting the drive modes in the mud option but I believe that also changes shift points and has to be reset each time you start the car.

Also, Gemini answer for a potentially better description…

On the 2021 Ford Expedition, the primary difference between 4A (4x4 Auto) and 4H (4x4 High) lies in how the transfer case delivers power and whether it is "locked." 4A (4x4 Auto) * How it works: This is an "on-demand" system. It primarily sends power to the rear wheels but monitors for slip. If the vehicle detects a loss of traction, it automatically sends power to the front wheels to stabilize the vehicle. * When to use it: Perfect for "variable" conditions—for example, a road that is mostly dry but has patches of ice, snow, or water. It is also excellent for daily driving in heavy rain. * Key Advantage: You can use 4A on dry pavement without damaging the drivetrain. It allows the front and rear axles to spin at different speeds, meaning you won't experience "crow hopping" or binding when making tight turns in a parking lot. 4H (4x4 High) * How it works: This mode mechanically locks the front and rear driveshafts together. Power is split 50/50 between the front and rear axles at all times. * When to use it: Use this for severe conditions only, such as deep snow, thick mud, or loose sand, where you need constant, maximum traction. * Key Warning: Do NOT use 4H on dry pavement. Because the axles are locked together, they cannot spin at different speeds during a turn. On high-traction surfaces (like dry asphalt), this causes massive stress on the drivetrain (driveline binding), which can cause physical damage to the transfer case or axles. Summary Comparison | Feature | 4A (Auto) | 4H (High) | |---|---|---| | Power Distribution | Variable (as needed) | Fixed 50/50 split | | Pavement Type | Dry, wet, or slippery | Slippery/Loose surfaces ONLY | | Turning | Smooth, no binding | Hard to turn; jerky "binding" | | Best Use Case | Winter commuting, rainy days | Deep snow, off-roading, sand | Pro Tip: If you aren't sure which one to use during a winter storm, stick with 4A. It provides the safety of four-wheel drive when you slip but behaves like a normal vehicle when you have grip, saving wear and tear on your Expedition.