Have to "raise the hood" and get in there to figure out the cause.
It's typically simple fix, just takes time to rummage around in there and find out what's rubbing on what then file, stone, or polish as needed.

Given your description, I'd suspect the the drawbar plunger & spring and the chief culprit. The plunger can be rough and/or lack a tapered end and sort of grind/rub across the spring coils. Likewise the spring coils can be hitching at the opening of the spring recess in the frame.

Secondly the drawbar itself may be rubbing on the frame interior somewhere. Inspect for any burrs or flashing on the frame. On the drawbar, the side flats and top edges may be stoned if needed. Pro-tip You can color the drawbar with dykem or a blue sharpie then reassemble. Cycle the trigger a bunch and anwhere the color is missing shows you where unauthorized rubbing has occurred

Cheers
Bill

__________________
Because they don't make a .46

Thanks very much Bill. Of course, I can't "raise the hood" on those guns that I didn't or couldn't buy (one was already sold awaiting pickup at the time), but I get your meaning. It's really a concern for the future and it helps a lot to know the possibilities (in terms of cause) and the potential for a reasonably quick & easy fix if I run into that issue again. If I am ever back up at the place that had that second "R4", I'll ask if I can field strip it. Not sure if they will allow it, but I won't know unless I ask.