They were discontinued, and since the 4004Cii/5 was also discontinued, it's been several years now that Rickenbacker don't offer 5-string basses of any model. So the 4003S/5 is sought after and prices on eBay are going steadily up.
S/5's were basically modified 4003S's (the non-deluxe model, with dot markers, no binding on the neck and body, and mono output). Tailpieces are modified 4-string units, with a 5th hole drilled between the A and D string holes. The necks were the same width (or almost the same) as the 4-string 4003, so string spacing is tighter than other modern 5-stringers. Some owners have modified the bridge to slightly wider spacing, by using new saddles and putting the slots off-center.
Over at the RickResource forum there's
a picture thread on S/5's.