I don't have any experience with the F4, but just from looking at the manual: I suggest that you put the camera in a changing bag, open the back, remove the film and wind it into the cassette by hand. Then you both save the film and can test the operation of the R1 lever freely without any tension from partly wound film, including trying to fire the shutter and see if that frees it.
If you don't have a changing bag or a darkroom, buy a changing bag. They will save your tuckus many times over.