Cut a darkslide in half (actually just a little more than half) and do two 2x5 images on each 4x5 piece of film. I do this with 8x10 all the time.
Or just crop the 4x5 film, using a mask as already suggested -- it is cheap.
Below is from using the modified darkslide on an 8x10...
Last Light, Yosemite Valley -- scanned platinum/palladium print