There are two general styles of NEMA 14 receptacle in wide use -- one is the conventionally yoked style that is used with a separate junction box in flush mount applications, but can also be used in a suitable surface-mount j-box:
while the other is a surface-mount-only style that comes with a specially-fitted metal junction box with a non-standard hole pattern and conduit knockouts in the back and bottom:
In a case where the grounding conductor for the circuit is a conduit, the former type of receptacle can be grounded in any way possible for a regular duplex NEMA 5 -- either through the yoke if permitted by Code, or by way of a bonding jumper from the receptacle ground screw to the box. However, the latter style of receptacle does not appear to have facilities in the box for a grounding jumper to be screwed to the box, and I am not sure if there is a "yoke" per se in that design that provides earthing continuity from the receptacle earth terminal to the box. This leaves me with the question: short of pulling a redundant ground wire through the EMT, how can I ground an integral-box-type NEMA 14 to a run of EMT? Or is the ground prong already set up to be electrically continuous to the backbox in such a receptacle design?