I'm guessing it is a setting relating to monitor display over HDMI to avoid playback jitter on screens that support 24Hz.
Edit: It also seems that according to current standards 4K can only be transmitted over HDMI at 24Hz, whereas UHD can/will go higher in the HDMI 2.0 spec.
Edit2: Having looked at the GH4 instructions, 24Hz (Cinema), 50Hz (PAL) and 59.94Hz (NTSC) are just system frequency settings to keep recording framerates divided into their specific regions. 4K records at exactly 24fps (only available in 24Hz mode) whereas UHD can record at 23.98fps, 24fps, 25fps or 29.97fps depending on what the system frequency is set to.