You don't need to put your camera on the back of your 4x5 as asked in the OP. If the "correct" exposure for the scene you want to photograph is 1/125@f/11, however determined, and you use 1/125@f/11, you'll have the right exposure, bellows factor apart (see below).

If in order to determine your exposure you use a FSLR, or a DSLR, it's enough that you point the FSLR/DSLR to measure the same scene. You don't need the camera to be inside or behind the LF camera. Just have it frame the same scene from roughly the same point of view.

If your goal is to have an "internal" light meter that will take into account the bellows factor of your 4x5, then the answer to your question in #26 I suppose is no (it's 3 am so I'm not sure of anything ).

Suppose you have a bellows factor of 1 stop. Your DSLR at the focal plane, supposing you manage to have it not measure any stray light, tells you 1/125@f/11. As your bellows factor is 1 stop, that means that your LF lens must be set at 1/125@f/8 to give the "correct" exposure that you have determined being 1/125@f/11.

This means that you have to calculate and know in any case the bellows factor, and "factor it" in the exposure calculation, which probably defeats the entire exercise.

If you know the bellows factor, you can take a measure with your DSLR of the scene, use this as your base exposure, compensate it for the bellows factor, and set the compensated exposure on your LF lens.