Lenses for 35 mm still cameras are mounted in helical focusing mounts. Look at one. Or look here http://www.bhphotovideo.com/c/search...-_-Photography to see some made for LF lenses. The difference between small format and large format helical focusing mounts mounts is that the lens is buried in the small format mount and is hung in front of the LF mount. It is possible, barely, to hack a small format lens (remove the glass, ... ) so it will serve as a focusing mount for a large format lens. Be aware that short lenses that cover 4x5 are very expensive.

There are a number of 4x5 pinhole cameras around.

Especially if I can replace the lens with a pinhole, in which case everything will be in focus
Wrong, wrong, wrong. With a pinhole, nothing is in sharp focus. Some like the effect.