D4, D3, 2, 1 are gear sets for certain road conditions. Most of the time you would drive with D4 which will as normal have you use all four gears.
Using 2 would mostly be for hilly terrains, as the 2nd gear is one of the tallest gears.
using 1 & D3 these gears will only allow you to drive up to 3rd (D3) & 1st (1).
D1 is for climbing steep hill or driving in conditions like driving slow in snow.
If you don't intend on going to through any hilly areas anytime soon just use the D4 and drive as normal.