There are several options for shooting 4x10 on an 8x10 covered here:

I do something similar but with my 4x5. I cut the main section of a dark slide in half and swap that out with the regular dark slide once the slide is in the camera and the shutter is closed down. This works particularly well if you have a rotating back. Chamonix sells pre-made solutions for 4x5, 5x7 and 8x10 here: