D allows all forward gears to be selected. 4 only allows 1, 2, 3 or 4 to be selected. It doesn’t alter the change pattern or the revs it needs before changing gear. Sport mode does that, by holding gears longer so revs increase more before changing up, which gives betterer acceleration. 4 is suggested on winding roads as it won’t let the auto select 5th (at 40mph for v6) when it starts to relax at steady speeds. Hence you don’t get continuous changes 5 - 4 - 5 as you slow before a corner then speed up afterwards. I never bother with 4 but have selected it by mistake occasionally. It's down to personal choice.