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Like you, I always used the folk 3 finger G for years and now i have gotten used to the 4 finger style. My fingering, from the treble E is pinky ,then ring finger on the 3rd fret of the treble E and B strings, index is on A/2 and the middle is fretting the Bass E/3.
For the D chord my ring finger does not move from the B/3. The index goes to the to G/s and middle to to E/2. That leaves my pinky available to finger the D/4 F# and make a fuller sounding D chord, especially if you hammer it onto that fret once and a while.
if I am using the hammer on I rotate my index finger to cover the the 3 treble strings, the middle finger is now fretting B/3 and the Ring finger can hammer onto the D/4 F# note for either and fuller D chord or also a Bm7th chord partial chord which is a great transition chord to either the Am7 or C chord.
A good progression Is the G chord to the D chord to the Bm7th for a beat or 2 and then the Am7 to the full C chord, Back to the starting G chord.