The WB does not have panning, which opinions differ on if that's valuable (depending on use case). Panning as a feature is probably useful when you start getting into competitions where shooting off odd surfaces don't allow for traditional bipod setups. The downside is panning adds weight, height, and one more moveable interface likely reduced stability slightly. Panning is probably more useful in something like PRS or some hunting situations, but for just typical range use, I never use it. They are both big bipods.
It's hard to tell which is more stable. The WB-4 without panning lets the rifle sit lower in the cradle between the legs, which increases stability, but the FC-4 has wider legs, so it could be a wash stability wise, with the FC-4 being just a larger/wider bipod. If they made the FC-5 without panning it would be even more stable. Which is probably what the HD-50 is but there's no 45 degree leg option on the HD50 so that's a deal killer for me. Neither are small bipods so for PRS etc. neither is probably a good choice.
Height is harder, the Accutac's don't have much height adjustment compared to other bipod options, for bench use it seems like most guys prefer the "4" series, but if you like running the legs at 45 deg, or you're a bigger guy prone the "5" series might be better. It also depends on the rifle and it's attachment. I know Frank has said in the past many guys run their bipods too low for good positioning. I think this is one of the biggest limitations of the Accutac line, very little hight adjustment in the bipod legs, but probably helps with stability.
If you have an FC-4 and it's height is fine, I might try a WB-5 for a couple reasons. First the WB lets the rifle sit lower without panning so you need a bit more bipod height to get to the same place with a WB as an FC. The other is that you can swap legs between 4 and 5 series. Which means if you have an FC-4 now, and buy a WB-5 you can mix/match the two leg lengths to get exactly what you want.
If I already had an FC-4 unless it was not working for bench/prone use I'm not sure I'd try out the WB series, if I did it would be with the intention that I'd pick the one I liked the most and dump the other one.