I sell TV's part time, and we always say the same thing - use a monitor as a monitor and a TV as a TV.
You can get a 30in or thereabouts 4K monitor, and it will work very well for gaming, etc. Its designed to be very close to you, have the inputs you want, etc.
A TV, less so. Yes, you can game, and there will be a game mode on all TV's. but you have to think about processing and pixel density.
So a 4K set with an Xbox, in a living room great. But if I'm writing a letter, then a monitor is going to be a lot more comfortable.
27in Mac desplay? Very nice - keep it. A Samsung 40in 6400 is roughly £549, and is excellent value as a decent entry level 4K set. You can certainly use it for all sort of smart things, but both is better than one.
As for distance - I want it to be perfect. There is a real macho thing where people want to sit next to the biggest set possible - its not like buying bog roll - its not about bulk for your money, nor is it like eating the hottest vindaloo. I think of it likes shoes or a suit - it should fit and be comfortable.
its roughly 1-1.5 x size of screen in the cinema if your watching 4K, but thats a bit loomy in a living room. I normally advise customers about 2 x size of screen away, perhaps 2.5 (most customers are very afraid of overwhelming a room). Remember that a 4K set has 8m points of light (but that does not mean its any good as a TV, refresh rate and black levels are what your looking for,) and on an HD source, its upscaling by a factor of 4. Thats OK, but you dont want to get too close. And watching SD on a 4K screen isn't fantastic - even a good one is going to struggle.
BTW - gamers will often refer to 60hz/120hz refresh rates, because the websites that mention the sort of stuff that gamers need to know (like lag times) tend to be US ones. The UK has 50hz/100hz/200hz screens (under the bonnet - the numbers often quoted are arguably meant to confuse), whereas the equivalent US ones will be 60hz/120hz, etc. I had a customer the other day talking about 120hz, and I guessed where he'd been looking.
Bottom line with TV's - the better the picture, the more it costs. Having said that, you can get something pretty decent from the big 4 brands for much less than ever before.