We've had this discussion both in-house, with commercial clients, as well as with Pix4D and from what we are aware, still images on the X4S use a mechanical (or global) shutter while video is using a rolling shutter. Pix4D also has a linear rolling shutter correction built-in that was for the Phantom 4 but it should not need to be used with the X4S. If for some reason you were using frames from video on the X4S you may run into rolling shutter issues that may require the correction with Pix4D.
Hope this information helps, don't hesitate to reach out with any further questions. Safe flying.