I have to say I'm still not personally convinced about 3D, but that's a whole different discussion!!
3D was a gimmick to sell new TV's, 4K is a legitimate technological improvement and I'm all for it!
4K is going to use up 4 times the bandwidth that HD does now. And 8K will use 16 times the HD bandwidth. In terms of practicality for transmission, I think 4K isn't practical, for either terrestrial or satellite transmission.
That's true for 25p content, when uncompressed.
However it's actually double what you quote if you compare 50Hz HD - as we don't use 1080/50p, but instead 1080/50i currently (which is the same in bandwith terms as 540/50p). 4k will be 2160/50p, so you actually have an 8x bandwith increase in baseband terms?
On the other hand, 4k and 8k are unlikely to use the same H264 compression scheme as current HD broadcasts in Europe (the US, Japan and Australia use the far less efficient MPEG2 for HD on some platforms) H264/HEVC should increase compression efficiency significantly, so that could bring it back to 4x or 8x, or even less. I've seen 20-30Mbs quoted as figures required for 4k using HEVC - and this is with first gen encoders. It may end up being less than this. You'd probably get 2 or 3 HEVC streams on a decent DVB-S2 Ku band transponder. Not a million miles away from the bitrates used for HD when it launched with sub-par H264 encoders.
The H264 2160/50p tests on Eutelsat are around 32Mbs, using 4 x 1080/50p streams at 8Mbs each (effectively taking a large proportion of a single transponder)
For satellite, the Ka band may make sense for 4k (though fitting a new LNB may not be a popular move). OTA is always going to be tricky as the available bandwith disappears to 4G/5G mobile data/telephony.
I think 4k has a much better chance of success than stereoscopic 3D - but am still not convinced how mass-market it will be. After all, people are still happily watching DVDs instead of Blu-rays on their massive flat-screens, even though they look pretty dreadful upscaled.
Having seen 4k and 8k content I am a fan - but not sure how big the draw will be outside of sport, movies and events.