Is a 4p4c rj22 plug physically compatable with a 6p6c jack?
An RJ-22 plug will fit into a RJ-11 or RJ-12 socket but the RJ-22 plug is much more narrow than the RJ-11 /RJ-12 plugs. This will lead to an unreliable connection because the wire pins in the socket will be bent as the plug is twisted sideways.

As a person who works with these connectors regularly, I would assume that a RJ-12 socket would be used only for a RJ-11 or RJ-12 plug and never plug a RJ-22 plug into it. 

In other words, I think that using a RJ-12 socket for a RJ-22 plug is a really bad idea.