I've been searching for an answer to this but haven't been able to find anything definitive.
I use an Orah 4i 360 livestream video camera. It sends the camera video to a stitching box. The stitching box sends out 4K (4096x2160) H.264 prestitched 360 video over RTMP. It also can record to internal SD card. But it can't record internally at the same time it is outputting over RTMP.
In order to record the livestream, you have to use the HDMI 2.0 output, which also sends the the same H.264 mp4 video, but without the RTMP 360 control data.
I need to record the 4K mp4 video from HDMI to an external HD or SD card. Since it is already encoded mp4, even though 4K, it doesn't really need the data record rate that something like an Atomos Ninja Inferno or Black Magic options. They record RAW uncompressed video, and you pay for that. I only need to record mp4, even though 4K from HDMI.
So, does anyone know of a cheaper solution than Atomos has to record 4K HDMI mp4 to an external HD or SD card?