RA-4 paper is generally panchromatic, so you *should* get a B&W silver image -- however, the relative sensitivities of the various color layers may not be a good match for what you'd expect, and there will likely be some color cast or other problems with the resulting print.
You're correct, B&W materials in any current color process will come out with no image at all; there's no dye to replace the silver removed by the bleach, because they don't have couplers to hold the dyes presented by color developers.
Cibachrome chemistry is essentially a B&W print developer followed by a dye-destruction bleach -- where the bleach removes developed silver, it also destroys the dye in that layer, then the fixer removes the remaining halide leaving a three layer direct dye positive. RA-4 paper is completely incompatible with this process; you'll most likely get blank paper with whatever color cast you'd get by putting it in common B&W fixer. Cibachrome paper in RA-4 chemistry will give solid black -- nothing in the chemistry will destroy the Cibachrome dyes, so you'll wind up with three full density dye layers over the whole print.
You *can* reverse RA-4 paper, BTW -- expose it to your slide, process in B&W developer, stop, wash, exposed to light to fully fog, then put it through the regular RA-4 process. You'll get some odd colors, but might be able to find a filter pack that works. You *might* be able to get a negative print from Cibachrome, but you'd have to develop, bleach in something like a B&W reversal bleach (must dissolve the silver without either dissolving the unexposed halide or rehalogenating the developed silver), then fog and process normally.
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