The X3 had a fixed aperture so that value was not available to change. The X4S is a much more complicated camera that has access to the ability to change the actual amount of light that is coming in the sensor instead of changing the sensor's gain (which causes noise). Changing the aperture is better.
The focal length is not changing. There may be small differences in the focal quality due to the aperture changes but as long as the focus is pulled when the aperture is at the smallest values possible F1-3 then when the number goes to higher numbers it will still be in focus.
This is just how the shutter priority mode works in the camera.