Probably everyone knew about how to do this except me but I've just put an hour of 4K on a 25gb Blu-ray as data using Nero. for what it's worth
The 4k mp4 won't play via Blu-ray player
Just a cheap way to deliver an hour of 4k what with discs down to 0.50 or there abouts

It will only be playable in a Windows or Mac device with a BD-ROM drive attached.

Now that 4K UltraHD Blu-ray discs and players are shipping, I wonder when 4K burners and authoring systems will come down to the prosumer level.

Regarding 4K on Blu-ray Discs, you can get an external BD ROM drive for $35. Then again, you can get a 32 GB MicroSD card for $12. But a blank BD-R disc costs only about 40 cents. If you send somebody three discs and a drive, you've hit the break even point.

But to me the more exiting thing will be to edit, author, and playback HDR. To do that I'll either need to stream to an HDR TV or burn a 4K UHD BD. Yeah, it's a reach today, but someday HDR will be standard practice.