You can figure this out from the ratio of the film sizes, which is easy in this case because 6x7 (55x69mm) is basically the same aspect ratio as 4x5 film (95x115mm), and the ratio of the two sizes is about 115/69=1.66. If you're familiar with 6x7 and want to think about 4x5 options, keep that 1.66 number in mind for all your conversions.
To get the same field of view, you need 110*1.66 = 180mm lens. And for the same DOF, f/(2.8*1.66) = f/4.5. And because of the smaller relative aperture, you need 1.66^2 = 2.8x (about 1.5 stops) more illumination, exposure time or sensitivity.
f/4.5 lenses are not real common on 4x5 but they do exist, e.g. Xenars. Most people just cheap out and buy modern f/5.6 lenses and ignore that last half-stop - you won't be able to shoot in as low light as you can with the RZ; you will need 2 stops more light (f/5.6 vs f/2.8) than the RZ does. That's the tradeoff with bigger film.
If you go to 8x10 that's a doubling, so it's 2 more stops of light required over what 4x5 needs.
Great explanation. Thanks polyglot. Am looking at sinar f2 at the moment. Indoor portrait with the odd outdoor shoot.