Wiring 3P+N Device to 3P+N+E Connection
Yes, that's totally fine.  Simply do not connect anything to the N connector on the plug.  

You may also change the wall receptacle to 3P+E if you prefer; in which case firmly cap off the neutral wire exactly the way you would an unused hot.  You must wire neutrals as if they are hot, because fairly trivial wiring issues can cause them to become hot.  

Cost it either way, whichever works the best for you. 

An RCD is not going to care which one you do.  It is looking for currents to be equal (cancel each other out) on all 3 hots + 1 neutral; imagine if all 4 wires were wound together around a current transformer; the net current should be zero.  

RCD devices are not concerned with ground. Ground bypasses the RCD device entirely; go look.  In fact on USA GFCIs, every GFCI made does not have a ground terminal, except for GFCI+receptacle combo devices, and they only have ground for the sockets. (it doesn't even go to the GFCI part).