Sony has announced the new firmware v1.1 to fix the inability of the camera to record 4K internally at the same time as outputting an image to externally via the HDMI port.
Got to put my hand up and say I have never tested this out on my FS5 but for those using 4K 25p 8bit I suspect its been a frustrating time.
Sonys download site tells us the following…
1.) External output and face detection support when recording in XAVC QFHD (4K mode)
You can get the download here : http://www.sony.co.uk/pro/support/software/SW_122115_PSG/1