SD or 480p doesn't have much info to upscale well. Most well-reputed brands & even big Chinese ones have good upscaler for 720p & 1080p.
I have personally checked Samsung, LG & Panasonic 4K TVs, SD channel doesn't look that good. Due to bigger screen area, it looks worse in larger TVs. You can somewhat compensate by sitting a bit far away, but as I said, the reality is 480p doesn't have enough information.
You can watch SD channels in 4K TV, but how good it looks is a bit subjective based on the person. Like I think SD looks bad, but its good enough for my parents in the 55" 4K TV they have at home.