You can only get it through their most premium Sky Q package and then you have to pay extra for the additional 4k content on top, it's just ludicrous - but they know elitists will pay. £10-20 for say 4/5 4k channels wouldn't cover it? E.g. Sky One, HBO, whatever else is 'premium' on Sky these days.
but I doubt if they would get enough subs. I had 4k as a F&F deal & full package for £48 a year.
To put 4 or 5 channels in 4k would be the equivalent bandwidth (if not more) of all the Sky HD Channels put together (Sky as in movies, sports, etc. - not inc. BBC, ITV, etc.).
Bandwidth costs money - which is why they will not do it until they think they would get enough subscribers.
It took Sky 3 years or so to launch HD channels after the launch of HD TVs, but that came around at the same time Flat Screens dropped in price.
Still think we are a year or 2 away from Sky going 4k route via satellite imo... and maybe why Sky Q is called Sky Q silver atm?