Tough choice.........I own and like examples of both. They are similar, but have some distinct differences.
The 645 is slightly lighter than my 4506-1's. Perhaps slightly thinner as well.......I haven't put calipers on them.
The 645 had a very nice grip frame with the capability to use different grip panels. I like the Pachmayrs on it. I do wish the 645 had the undercut trigger guard like the 4506 as it lets the pistol sit a little lower in the hand.
The 4506-1 has a better trigger than the 645. Although I'm told that Novak's will put a very nice trigger on the 645 and undercut the trigger guard as well.
Reliability and accuracy are a wash. Both are bet your life reliable and tack driving accurate.
I like the look of the adjustable sight 4506-1 and I like the 645 with Novak sights just as much. So........I guess I'm no help to you at all.
The 645 was made for a shorter period of time and in fewer numbers than the 4506. So if there is a 645 in front of you, I would advise you buy it..........first.
Regards 18DAI