D4 limits the transmission to gears 1 throught 4. I rarely use it, but it would be useful for towing in hilly county. I use it when I want quicker acceleration, like an interstate on ramp. It will give you more engine braking when going down hill than if you were just in D. No different than downshifting a manual tranny for quicker acceleration or engine braking, only you can only limit it to the lower 4 gears.The 2020 EX-L Owners Manual, Shift lever position, says "D4 Press the D4 button while the shiift lever is in D. Use when: Going up or down hills."
Can some one explain, I thought it was an Over Drive .