B&W Prints from B&W negs on RA4 paper is indeed possible. I use an LPL 6700 enlarger with the 35mm carrier but on to of the carrier I lay a square of 120 film that came from a part exposed but developed C41 film. I don't actually have any problem with contrast and when using Fuji Crystal Archive paper a filtration of 25magenta and 35 yellow gives me a print that is very similar in tone to Ilford warm tone B&W paper. Those filtration values of course will alter according to your darkroom and RA4 chemicals used, but they will be a good starting point. The exposure times tend to be a little longer than for straight forward RA4 with C41 film, I assume due to the extra film density and any tint in the B&W film base..