In my experience, even between brand new motors, and even from high quality brands, it's very common to see some spin more or less freely than others. Not sure if that's an accurate way to judge wear. And if it was, the fact that one of yours is looser than the others would suggest that the other 3 were replaced with newer ones, which seems less likely on a referb than having to replace a single motor.
Motor wear is definitely concerning. It's the one wear item, it's hard to really judge lifespan, replacing them arbitrarily is expensive and/or difficult, and a pending failure could go from undetectable to total within the duration of a single flight.
I have yet to have a drone just fall from the sky (from something other than my own tomfoolery), but I know it's just a matter of time!