I've got a 4x5 back for my Wehman 8x10. I bought it used at a good price off Ebay. The reduction back is great for the occasional 4x5 color image. Color 8x10 film is expensive! I shoot 8x10 b&w.

I guess it all depends upon how much it costs and how much you will use it. If I were shooting a lot of 4x5 though I'd rather have a 4x5 camera in addition to my 8x10. Like said earlier, an 8x10 camera is pretty big to carry around to shoot 4x5.