I have a 4x5 back for my v8 which I bought , alas, to use polaroid Type 55.
While a 4x5 camera is far more portable than an 8x10, I can see some uses for a 4x5 back on an 8x10---for example if you want lots and lots of bellows
In general though, an 8x10 with a 4x5 back will not easily replace a 4x5 in the field (a studio set up is another matter.)
Since you are an admitted 'dorffer, unless your 4x5 is a jewel like v4, it is a 4x5 back on a 5x7 body, and you can easily put a 5x7 back on her, so except for the wieght of 5x7 film holders you are loosing nothing by shooting 5x7 while gaining a whole lot more detail and a more pleasing size contact print to boot!