For raw texture then 4K render and downscale is a wasted op. If you load a more detailed texture and sample 4 pixel locations that end up in the final render, then downscale to 1080p by merging those 4 pixels then it generates the same average that the next mipmap pre-computed (because each tier in the texture is quarter-area/half-size and so does just pre-compute the averages of 4 texels [texture pixels]).
Where it does have value is at the edges of polygons, where you’re not just getting the same as a mipmap, because it’s averaging over several different textures. So it’s a slight improvement (but mainly the visual benefit is super-sampling and avoiding aliasing). One of the other benefits is from shader aliasing. No longer are game engines just looking up texels and throwing them into the render buffer, they’re running complex lighting code and looking up several textures (shininess, colour, normal, smoothness) to calculate the resultant pixel value. Sometimes this leads to discontinuities in neighbouring pixels, creating aliasing inside a single polygon area. This aliasing is also improved by supersampling, just as polygon edges are. So it’s outdated to purely consider mipmaps as the complete solution to maximising quality inside a single polygon boundary (unless the shaders have all been constructed to avoid aliasing). Although how much that benefits from higher detailed textures is an open question (note that often some of the textures are lower res then others and there it does help to get more detail) so it could be that 4K rendering without “4K textures” would be just as effective in solving the shader aliasing issues.
A note: you are already used to massive supersampling to avoid aliasing. Whenever you watch CGI/modern animated movies then you’re enjoying the precision of rendering that brute-forces around aliasing issues. When you brought a Pixar movie on DVD you saw the benefit of rendering at a resolution far beyond the SD output that the DVD contained. Rendering internally at 4K for a 1080p TV is a proven upgrade, we know because we’ve been experiencing the benefits in offline-rendered CG content for decades.