That would have been produced years before the rb variant was the standard production model for all bbl. lengths - that came about in 1996 or so. So my next move in that case would be to check the butt under the grips and see if it is a factory RB or was a conversion. Comparing tool marks, finish, and mark placement should make this obvious. I checked the SCSW and no RB 4" variant is listed before the 1996 change, so I would strongly suspect an aftermarket conversion. It may be a special order, but one not documented yet. It could also be a 2.5" variant that was aftermarket "up-barelled." Yeah, sounds weird, but I've seen it done.
Closely examining the butt of the gun with the grips off is the next step.