A dedicated 6x17 would be more convenient (and more expensive). A 6x17 back for your 4x5 will be a lot cheaper and you will have more lens options.
I bought a 6x17 back and it works even with a 400mm lens (tele lenses only over about 200mm). Working with it is cumbersome and I have stopped using it (I think mine is for sale or trade if you are interested). Instead I use my 8x10 and crop the negatives when I want large panoramic images.
If money is no option I would recommend a dedicated 6x17
If weight and handling is no problem and you want all the options that you have with your 4x5 camera then a back could be useful