No. A firmware update would not reduce the 5D Mark IV’s native 4K crop in the normal sense. Crop factor comes from the physical sensor size relative to full-frame, and for 4K video the effective crop is determined by how the camera reads the sensor area. Firmware can crop more by using fewer pixels, but it cannot create a wider field of view than the hardware/sensor readout allows. So unless the camera were using a different sensor readout mode already built into the hardware, the original 4K crop behavior would remain essentially the same.