recording 4K H.264 HDMI 2.0 to external media?

hi.
i've got these 360 video stitching boxes that send out prestitched 4K H.264 mp4 video over HDMI 2.0.
i need to be able to record those HDMI streams in 4K.
the cheapest external recorder box that i can find that records 4K is the Atomos Ninja.
but it's over $1000.
part of the reason for that is that it can record in Prores and so on, and needs a more robust data transfer record rate than i actually need with an H.264 stream.
even in 4K (4096x216).
i'm looking for a cheaper solution than getting 3 Atomos or Black Magic recorder boxes.
i'd ideally like to just record to an external HD or fast SD card.
but it has to be independent of the USB pipeline on my laptop, since that is busy with the 360 video stuff.
i need something that goes direct from the HDMI port of the stitching box to the recorder without having to go through the computer.
multiple interfaces and boxes to make it work would be ok.

does anyone know of a cheaper solution than a high end 4K recorder that will just handle 4K mp4 video?

thanks,

randy
42 VR