99 ex coupe auto

I believe from what I read out of the owners manual, D3 just uses the first three gears of the four speed auto. D4 is like an overdrive for the Honda auto-tranny, it shifts into the next gear to make RPM's lower therfore saving fuel. When you floor it from a stand still, its shifts gears in order 1,2,3,4 etc. Shift point are approx at 40mph, 65mph, 105mph, then 4th gear takes over (these points are assuming flooring all the way through gears). I belive that all D2 does is shift it directly into second, hence the long crawl to vtec in D2 because of a taller gear, same as D1 but its only gear becomes first gear.
All in all, when "cruising", D4 will keep rpms down and save fuel, where say if you dont want to floor the accelerator to downshift to pass someone on the freeway, just shift it into D3.
You can shift freely without pressing the shift button between the gears of N, D4, and D3, but there is a bulit in protection preventing shifting into P,R or D2&1 from D4&3 and N.
Since autos have a torque converter, if its shifted into N while the car is moving, rpms will just rise, and when the car is shifted back into either D4 or D3, it will reengage.
I use D3 sometimes on the freeway to slow myself down (from say 80 to 70, just letting it "coast" in D3 without pressing on the gas) so I don't have to use brake or when gettin on a freeway so you dont have the tranny constanly shifting if its hard to merge.