1) That error means the data coming in FX3 is at a faster rate than what is read from the USB. This could be because the host application is slow and the processing inside FX3 is taking time., You may be adding headers to the data in Fx3 which might be causing delay.
2) 4K (16 bits) at 30 fps seems unlikely in Fx3. 3840 * 2160 pixels
3840 * 2160 * 16 bits * 30 fps = 3981312000 bits per second
Maximum camera reception supported in FX3 GPIF is is 32 bit * 100 Mhz = 3200000000.,
Also, processing inside FX3 will take time if you are adding headers. So it is not possible, 1920 * 1080 resolution at 30 fps has been tested for raw mode, We have not tested higher resolutions.
This KBA explains about the reason for the error "Invalid sequence (error code 71)" and how do you recover from this error.