Why can't they offer a dual p, or a P/MM setup?
As many options as Carvin already gives us, we all want more options, and we all have that dream bass in our head that we'd like Carvin to make for us.
It sounds like Carvin is only offering one pickup choice at the bridge posotion, a specially wound J-99, because that is what matches up correctly with the split coil.
Think of it this way, let's say Carvin offered the PB4/5 with the option of any other pickup in their line at the bridge position. Suppose most of those pickup combinations performed poorly in the eyes of the consumer. (Remember that the bridge pickup will need to match up well in terms of output with the split P at the neck.) That would represent a huge amount of money and time on R&D down the tubes, plus a big black eye for the company.
The prudent thing for Carvin to do, as well, is to limit the new P-style pickup to the PB series until it has proven itself before offering it in other basses.
P.S. I would have ordered a PB4 by now, but I still can't decide on the silly aesthetic considerations of color of finish and pickguard.