Since I bought one of the "early" ones with the phillips screw in the decocker and a "later" one with the hex screw, I have noticed several other differences.
First, the backstrap on the early gun is metal while on the later one appears (and feels) to be plastic.
Next, I notice that the early gun has a "half cock" notch on the hammer that the later one does not. I have noticed that if, after decocking, the hammer is manually cocked to the notch, the DA trigger travel is reduced by about a quarter of an inch. To me this shorter DA pull seems to be an advantage for first shot placement.

My understanding is the 645s had some such merits that disappeared with the 4506s -- and for that reason, some guys still prefer the 645.

I don't know firsthand, though.