Definitely you need 5x4 film inserts, they do crop up time to time, I have quite a few in various sizes, however there are some variation and while the outside dimensions are standard some are thicker than others - designed for specific plate holders. Usually the thicker ones have a pattern or large ridges pressed into the back to give rigidity.

I'd need to check but I think the septums from the Grafmatic backs are ever so slightly too wide, I have 3 British Wray Grafmatic backs but they should be identical to the US ones (they were made under license like the Dallmeyer made Parkard shutters).