I have 4 threaded holes in an AMPS pattern, and I'd like to mount a Quick-Grip phone holder directly to the AMPS plate, without the use if ball arms, so it sits as low as possible, tight to the AMPS plate. Do I need this adapter plate, or will the 2 holes in the Quick-Grip meant to attach a ball line up with 2 holes in the AMPS Plate? Is there some other solution to mounting the Quick-Grip as flat as possible to a plate with AMPS pattern hiles already in it? I don't need it to be adjustable. Just want to mount a Quick-Grip phone holder as tight as possible to an existing plate with AMPS hole pattern.

Yes, you can attach a Quick-Grip style phone holder to either a 2-hole AMPS pattern like the diamond ball base (RAM-B-238U) or a plate that has a 4-hole AMPS pattern.  The 2-hole pattern is the diagonal between either of the two corners of the round or rectangular 4 hole AMPS plates.