10x15cm is the old German "Postcard Size" (or "Postkartenformat", to be precise).
Since 4 inches is 10.16cm, 4x5" can be expected to be about 1.6mm wider. It should be within the tolerance of the film holders if the holders are on the narrow side and the film is on the wide side. Slightly better chance of success than with 5x7" film in 13x18cm holders, but I wouldn't recommend it.
It's much more fun to use a 10x15cm camera, or even a 13x18cm one with 10x15 inserts. I've got a strange old contraption which after much pondering turned out to be a 10x15 plate holder adapter for "regular" 13x18cm / 5x7" cameras. Rummaging around in the heap of boxed with "potentially useless stuff" I also found two 10x15cm plate holders with film inserts. So now all I need is the film...