Roll film holder
Medium-format roll film holders of many different manufacturers can be used with 4×5″ cameras. Usually for 6×7, 6×9 and 6×12 formats. The one on pictures is a Wista 6×9 roll film holder.
Cut film holders
Remember that there is no such thing as too many cut film holders.
Polaroid backs are just great. Polaroid is a unique medium, great to work with.
Accessories for large format cameras
Angle shutter release
They are quite inexpensive and the best is to have one for each lens, leaving it attached on the lens. A angle shutter release is a must for wide angle lenses on recessed lensboards.
Self-timer shutter release
A fully mechanical self-timer is useful sometimes.
Lens bags, made of soft cloth, are great to keep your lenses and lensboards safe when carrying a lot of equipment inside the camera case.