I ordered some family bikes recently, and all were supposed to come specced with RD-M6000 medium cage rear derailleurs. The first one out of the box has an RD-M4120, which is a cheaper model, as far as I can tell, and doesn't include a gray clutch switch.
The next one had the RD-M6000, but in setting the bike up, it became clear that the M4120 was a lot easier to get dialed in. (I've adjusted plenty of rear derailleurs, and that first M6000 was stubborn for me -- and clutch was off.)
I'm wondering what the practical difference is in terms of performance and features. All these bikes will be used for fairly tame trail riding, snow, and some light (green dot) downhill runs. My first trail ride on the m4120 was perfectly fine, shifted well, didn't slap around much.