For me, no to 4KBD and No to these players!
Unless 4KBD is recorded on 100GB discs, it'll be a lower bitrate per pixel, possibly decreasing picture quality regardless of resolution. You'd be better off upscaling existing BDs to 4K. Not much has been said about audio specs for 4KBDs. Also, titles up to the present won't be remastered in true 4K--4KBD will only apply to future films.
As far as these players are concerned, if you own a receiver with an ESS Sabre ES9006 or ES9018 DAC, these players would be a waste, especially for stereo analog output only! What Pioneer really needs to do is add to their lineup a stepup player from the BDP-80FD and BDP-170, yet a stepdown model from the BDP-85FD, replacing the BDP-52FD, BDP-53FD, BDP-62FD, BDP-440, BDP-450, and BDP-LX55. Right now, I'm using a BDP-52FD with the new SC-81 A/
V receiver and I couldn't be happier. I just wish they'd get the bugs out of bitstreaming hi-res formats-occasional skips and audio dropouts.