Bear in mind Gizmo can’t get fibre broadband so unless (s)he’s right on top of the exchange ADSL isn’t likely to deliver 4k on amazon or Netflix but HD might be fine and will look good.
SkyQ 4k content (which has no HDR* support yet) is limited to football, F1 and a paltry selection of movies. It’s good but not enough content yet. At least SkyQ is a much better system overall so I don’t feel too ripped off.
FreeviewHD isn’t quite as nice as SkyHD - lower bandwidth - but it will still look good on one of these screens.
A 4k Blu-ray player (I use an Xbox one S because it was the cheapest option) will bring out the best.
And the demos in the shops are sometimes run off USB sticks delivering quality you can’t even get from a 4K UHD Blu-ray. They do look great though.
* HDR is the broader contrast and colour spectrum that really gets the best out of these screens. Blu-ray UHD, Amazon, Netflix and appleTV 4K support it but sky and freeview don’t yet. iPlayer just has Blue planet2 for about another 3 weeks but that needs at least 24Mb of broadband action. Dolby Vision mentioned above is an even better version of HDR. Not all players, disks and TVs support it.