After reading through the MK3S to MK4 upgrade guide, I believe it is because the MK4 has one more wire: The yellow green grounding wire. There isn't room for it on the quick release connector. But I can't see any other reason preventing use of this for the MK4. If you want to try, you'd have to loop the grounding wire through to the outside of the enclosure some other way. To avoid having to disconnect it from the PSU every time you want to remove the printer (thus ruining the point of quick-connect) you might be able to make do with the scew-and-nut extension between two ground wires as described in the MK3S-to-MK4 instructions for the silver PSU. It's not as quick as a single connector, but I'd rather have my grounding wires secured firmly with hardware than mcguyvering some flimsier extension. (I'm sure there is a proper quick but safe crimp connector for single wires that is suitable for ground, but I'm not the right person to figure that out.) Though maybe the power panic connector is different too? You could of course crimp the right pins onto the new power panic cable and re-pin the printer-side connector to fix that. Dunno what connector type this is, though. Note: Don't take any of this as advice and do not attempt it if you don't know what you're doing. These connections are important for safety, so it's essential that they can't break without your knowledge because you won't notice before it's too late. EDIT: You can now ignore all my ill-gotten advice since there is a proper MK4-model quick release cable available.
