No experience with the 645 but the 4506 was a good pistol. Trigger was never as good as a 1911 and they're bulky and shoot a cartridge that's kind of pointless when not used in a 1911 but they're solid guns and run well.
I think the 1911 is a better pistol (assuming we're describing actual 1911s built to spec, not the various 1911-shaped guns built by god knows who) but if you compare Colts manufactured at the same time as the 4506 to these...the machining is much better on the S&W. Of course those are the dark ages of Colt, and the Colts from that era still run great, but inside they're ugly as hell. Sometimes outside, too. I have never seen a bad example of a 4506.
I think the 1911 is a better pistol (assuming we're describing actual 1911s built to spec, not the various 1911-shaped guns built by god knows who) but if you compare Colts manufactured at the same time as the 4506 to these...the machining is much better on the S&W. Of course those are the dark ages of Colt, and the Colts from that era still run great, but inside they're ugly as hell. Sometimes outside, too. I have never seen a bad example of a 4506.