You can shoot with shutter speeds of 1/100 second and slower with the R4 using electronic flash. The X setting is for electronic flash synchronization and is electronically timed for 1/100 second. Shutter speeds faster than 1/100 will not expose the whole frame at the instant the flash fires, leaving you with a partially blank frame. Shutter speeds of 1/100 and slower, including B all leave the film completely uncovered by the shutter at the instant the flash fires. Longer exposure times will add more ambient light to the image if that's what you want. The difference between X and 1/100 is that 1/100 is a mechanically timed speed for using the camera if your batteries die.
You'll most likely want to use the X setting for studio work. Some flash systems designed specifically to work with the R4 will automatically set the camera to X when mounted on the hot shoe and turned on. I'm not aware of a studio flash system that would do that, so you'll need to set it to X yourself.
Lee