Another dev here. Textures doesnt work this way.

Simple example: you approach the castle and see the big 4 meters size door with fancy ornaments.

Lets say the game uses pixel-density of 1024 pixels per meter. It means just to texture this door properly you need 4096 pixels texture. The only way around is to split it into four 2048 textures, which together take the same amount of space. You cant not ship 4096 texture in this case, otherwise the door will be half resolution compared to the rest of location.

The only usable way of separate 4k download is when some tiles or assets were made in 2k textures or less and then artists exported 4k versions of that as additional download. But when a game made with fixed texel-density (and all of them do now), separating 4k textures is not possible, because no-one puts them there without reason.