Why Aren't There More 4x5 Reflex Cameras?

Shooting 4x5 portraits I can't help but wonder what it must be like not to lose sight of the subject after the film is in place ... I've read (perhaps here) that some value this uncertainty, or just don't liking being able to continually focus.

Has anyone used 4x5 in Super D, Gowlandflex, ART-FLEX, or Arca Swiss Reflex? Any opinions on SLR vs TLR? Does the Super D's mirror function automatically, or does it need to be engaged and disengaged manually - (if so, is this still better then having no continuous focusing ability)?

:D

I'm dreaming of making one since they seem expensive and hard to find. Any recommendations on sources of information describing angles of mirrors, focal plane specifics, correcting parallax, and other considerations?

:confused: