Seems to me it'd be the same RC substrate as present on RC B&W papers.
Originally Posted by MattKing
While my understanding of C41 masking is a little shaky, I don't believe it's necessary in order to get a balanced exposure from RA4 - see the Rollei maskless film for example. The orange mask is not a uniform filter layer but rather a dye layer that gets produced during development to counteract shortcomings spectral sensitivity of the cyan dye. You can have maskless C41 film, it will merely have poorer colour accuracy.
However CTO (daylight->tungsten) conversion filter might be beneficial since RA4 is tungsten-balanced for use in an enlarger. Unfiltered use in daylight would (I imagine) hit the blue layers much more strongly than you want.