...and others are saying with scaling down you lose quality...
What the? Scaling up loses quality, not scaling down. But, it depends on what you are using to scale down images, but usually, it's way better to scale down images than scaling them up. If you are not using vector graphics, scaling will always have some loss no matter what you do to it. Making it larger wil make it look more pixelly and smaller may make it look more blurred.
My sprites are 4k, should I downsize them in the room or should I keep them high res?...
4k Sprites? You would have said 4k textures, I would have understood, but sprites? If you have a 16k or 32k TV, I would understand using 4k sprites. But to help you understand better, consider how big your sprites appear on a 4k screen because a 4k sprite means that you will probably see it take almost the entire screen. That's why textures, I understand because some people can decide to look soo close to an object to see it's detail in a game. But sprites, for 2D or even 3D games, usually, they don't really take more than 512 square if they are huge. So, as
@muki said, do the maths of the proportions drawn on your screen and it should give you how big you want your sprites to be because it will indeed take up a heck of a lot of juice even for a 4k capable machine.
I would also suggest you to consider who will play your game and if these players all have 4k capable computers. This will give you an idea of which resolution to adopt for your graphics too. If most of your players are Mr. and Mrs. everybody/anybody, then a good percentage have laptops and normal desktop rigs with integrated graphics cards or 2gb to 6gb graphics cards. Another good portion, most gamers, have cheap to medium-good gaming rigs from Core i5 to Core i7 with 4Gb - 8Gb graphics cards. And you will have the rich ones or those that spent for their computer-baby top with GTX 2080 in SLI with the latest CPU on the market that can easily render 8k.
As for what
@muki said also 4k sprite sheets looks more adequate than 4k individual sprites.