I don't have a GoPro 4, but my experience with the GoPro 3 was the following:
- Setting the GoPro to 720p120 seemed to get it to output a 720p60 signal over its HDMI port
- Setting the GoPro to 1080p60 (or 1080p30) would cause it to output either a valid 1080i60 signal or a messed up 1080p60 signal out of its HDMI port (depending on the device it is connected to)
I believe this is because there is some negotiation that happens with the HDMI output. So if the receiving device only says it supports 1080i, then that is what the GoPro will output. For example if I connect the GoPro directly over HDMI to the ATEM Television Studio (which only supports 1080i) this seems to cause the GoPro to send a 1080i signal which shows up fine on the ATEM.
If instead I connect the GoPro to one of the HDMI to SDI Battery Converters (or HDMI to SDI 4K Mini Converter) both of which support 1080p60, then the GoPro will output a 1080p60 signal over HDMI. The HDMI to SDI Converters then output a 1080p60 signal over their SDI outputs (3G SDI). On an ATEM TVS (which doesn't support 3G SDI) or an ATEM 4K set to 1080i mode, it will not see this 1080p signal.
Furthermore, this 1080p60 signal output by the GoPro seems to be messed up in some way, and even connected to a device that supports 1080p like an ATEM 4K (set to 1080p60), I get a flickering/intermittent signal (or sometimes no signal at all).
Some people have used a device like the Gefen HDMI Detective inserted between the GoPro and the target device to force the GoPro to output 1080i or whatever the desired format is. I don't have a GoPro 4 yet, so it's possible that its HDMI output is different, but at least with the GoPro 3, the best advice was getting things working was usually:
- Use 720p if you can (setting the GoPro to 720p @ 120fps for NTSC or 720p @ 100fps for PAL)
- If you need 1080 resolution, connect the GoPro to a device that will force it to output 1080i
If you have both a GoPro 3 and GoPro 4 it would be awesome if you could try the different configurations with both cameras and report back what works in each case (720p60, 1080i60, 1080p60).