4x isn’t worse but it does smoothen/weaken more up and because of this it’s not really a gain compared to 3x. So for me I get highest detail gain doing dual Starlight upscale. First Starlight to min. resolution and then second pass 2x. The result of this is 1920p (when your source was 480p 4:3)
Then I do 1x Iris MQ Auto pure (set rec. orig Details to 0, optional Deblur slider) + I choose as second model “Gaia”, in the same pass. Iris MQ brings more details to shine, because iris does some reconstruction too and does some denoise, Iris MQ keeps textures. Gaia brings more naturalness into it, softens, fades out lines, corners, and edges a little bit, thus compensating for Iris artifacts.
Unfortunately it is still not finished 
because the result is often to “hard” with halo effects. So I import it into Hybrid, upscale 1920p to 2160p (because 1920p is not handy) and I use Hybrid Dehalo filter (try “FineDehalo” at default Value), optional I add “Filmgrain” filter in the same step. I never wanted to go to 4k but there is no other way because downscaling is always a loss
Fits not for all sources, but when it works the result is stunning, sharp and highest detail gain. My problem now is, how to keep the finest gained structures and grain, using h264/265 remove some of it and this is really worse, I tend now using AV1 codec for storing my final results.
Warning: This workflow is horrible time-consuming (also the IrisMQ-Gaia part, happens about 2-3 fps), especially if you do everything with a lossless codec like I do (I have a total size of 18TB SSDs in my Computer), so I can’t recommend it to everyone.
