I'm glad you asked. Absolutely I do not require or even recommend a 4-finger D chord. Many people seem to think that D is "supposed to be" a 4-finger chord, but it actually sounds *better* as a 3-finger chord, leaving the 4th string open. The open string is a low D, after all, so that chord sounds fuller than a 4-finger D. I suggest using all but the ring finger, leaving it free to play the 4th string (or the 3rd) when and if the melody calls for it.
The 4-finger version I guess is often taught because unlike the 3-finger one, it's a movable position, usable at other locations as an F chord, G chord, etc.
In the key of G, the easy 2-finger D7 substitutes fine for a D. But that substitution doesn't work in the key of D, where the D7 is not interchangeable with D. BUT... there's an easy alternative D chord, a simple 2-finger D, like the 3- and 4-finger versions but where both the 1st string and the 4th are left open. That D chord is not the best-sounding one, but workable.
The key of D is well-worth getting handy with, generally needing only D, G, and A. It's the most workable alternative to playing in G, and is often used when the singer can't easily sing a particular song in G. That's why our "required chords" list is G, C, D, and A. On the banjo G and A are pretty darned easy, and the easiest D is easy too, so we have the bar set as low as possible to make it easiest for people to come and learn how to jam.