You need an inch of clearance under the groundglass to fit a Grafmatic. If you don't have enough room, it may be possible to add washers or spacers or a larger spring to the back to accommodate a Grafmatic without compromising the spring pressure for regular filmholders.
Here's how they work. You've got six septums in a box covered by a darkslide, and that box slides in and out of a larger box that remains stationary in the camera. When you're ready to expose a sheet, you pull the darkslide and push it back in. The septums are under spring tension, so the darkslide automatically slides under the top septum and in the process moves a small metal piece to reveal a red dot, showing that the film is ready, and you shouldn't remove the holder from the camera, or you will expose the sheet. You take the picture. Now you pull out the inner box, but the top sheet remains inside the larger box, inside the camera, and the darkslide is now on top of the stack, so that the new sheet on top of the stack is not exposed when the inner box is pulled out of the camera. Springs push the top sheet to the back of the holder, and you push the box back in, so it goes on top of the exposed sheet, which is now at the bottom of the stack, advancing the frame counter in the process.
It sounds complicated, but it's ingeniously simple, and once you get the hang of it, you can shoot at the blazing speed of 1 fps with your 4x5" camera.