I think the ground glass on a 4x5 would be around the size the OP quoted.
My Graphic Super is about that size.
Agreed. The image area on a film is smaller than the actual film size. Actually, the film itself is smaller than it claims to be as it's specified in the size a glass plate would be but as originally film was put into metal holders (septums) to allow it to fit in glass plate holders, the film will be slightly smaller.
The measured size of 4.75 x 3.75 sounds just right for 5x4.