I like the idea of shooting only BW on the 4x5 with film and using the DB for colour.
Thats kind of what I'm doing. I'm 99% film, and love it. The problem is that for color reversal/slide film is very limited. While gorgeous, it has, at best, 5 stops of range, which make a lot of photographs impossible. I could shoot C41 color negative film which has a lot more dynamic range, but a)there is no way I'll every setup a home color darkroom for printing so I have to scan, and my setup for scanning (a Sinar Norma and X parts fashioned into a copy stand and a Pentax K-1ii with a 100mm macro lens) creates really difficult color balancing issues (the orange mask). My color sensitivity is very low. I'm strongly red/green colorblind. So using a DB (on a sinar Norma, or a AS F-Classic) lets me take color without the orange issues that need a lot less adjusting, and use film for B&W photos.
(of course I could send out my C41 for dev and scanning, which I do do for 120 film, when I take a camera on vacation, but for sheet film I really like to dev the same day, so everything is processed at home.)
Another point--my $2000 used P1 P65+ is about the same cost as 70-80 sheets of C41 8x10.