i have a 4k box with a 4k tv on picture setting in the bt box menu it’s default is set at 4k , should I leave it there or change it to 1080p? I only need 4k when I’m playing 4k content, does it make a difference to normal hd content if this setting remains on 4k uhd? Surely I don’t have to changing this setting depending on what I’m watching?
If you leave it in 4K, you'll get clearer images on the menus also your tv will probably upscale the other channels so you get a better picture on those too.
It all depends how good your eyes are. If you leave it set at 4K the box will do ALL the upscaling, but if you have it on 1080p your TV will do the upscaling and the TV will undoubtedly do a better job at upscaling as the upscaler in your TV will be matched to its screen (unless it is a VERY cheap tv). The downside is, as you state, if you want to watch 4K content you would need to change the setting on the box each time. I would suggest watch some 1080p with the box set to both outputs and see if you can notice a large difference.
It is a pain but I don't think there are many boxes out there that will output at default resolution, even the AppleTV4K which has match Frame Rate and Dynamic Range will still only output at a fixed resolution.