I'm in the same position - my Toshiba Regza is actually a very good HD performer but I would now like a 40" 4K TV as the bezels are now much smaller.
There is a shortage of decent screens available in this size from my research, as manufacturers seem to be ditching this size in preference for 43".
The only decent option I have found is this Samsung which I hope to be buying soon -
https://www.amazon.co.uk/dp/B07D791M...v_ov_lig_dp_it
If you wanted to buy second hand, the Panasonic TX40EX700B is now discontinued, but a very well reviewed 4K screen.