D4, that is overdrive in a 4 speed automatic. It should go into 4th gear at about 45 mph, although you may not feel it shift. The only reason to use D3 is if you are driving slow, like in city traffic, and the D4 keeps shifting in and out (from 4th to 3rd and back), as if it is "hunting gears". Most people just put it in D4 and drive away.
In your Lincoln LS, you will have 3 transmission options to choose from. The first is the SelectShift, which operates similar to a manual transmission but without the clutch. The other 2 are the D4 and D5 options that you are referring to. The number after the D is really a reference to how many gears it will shift through. D4 = 4 gears, D5 = 5 gears. All that being said, it really should operate closer to a 4 gear transmission with an overdrive gear when the car is in D5. Think of it like the "drive" and "overdrive" in other cars. I have heard that to maximize fuel economy, use D5 for highway driving and D4 for city driving.
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