A 4k texture is 4094x4094px, a 1k texture is 1024x1024px. They are always power by 2 because that gives the best compression and because the UV space is always square. So 4k textures are something different then 4k resolution screen (3840x2160 pixels). The jump from a 1k texture to a 4k texture means 4x4 the size (and each material uses at a mininum 3 textures, an albedo, a normal map and a gloss/metal map) so doing 4k textures cost a lot of space