It depends on your distance from the tv and the quality of the upscale / downscale process. However rendering at native resolution is always better than resampling in between.
It all comes down to the FOV you’re left with. The human eye can distinguish up to 120 pixels per degree (upper limit where people with great eyes can still ‘guess’ which image feels better), while 60 pixels per degree is regarded as 20/20 vision, good enough. (and is what all the recommended viewing distance charts are based on)
The formula for pixels per degree for a 16:9 screen is roughly
1080p: distance x 38.45 / diagonal
1440p: distance x 51.27 / diagonal
2160p: distance x 76.9 / diagonal
So for a 15.6" laptop screen, 24 inches away (2ft) gives you about 59 pixels per degree.
A 92" projector screen, 12ft away (144 inches) gives you about 60 pixels per degree.
Much bigger screen, same perceived resolution. And actually the laptop screen is ‘better’ in the sense that you can lean in to read the fine print without having to get up.
For the OP’s example going from 34 inch ultra wide 1440p to 43 inch 4K
As I understand it ultrawide 1440p is 3440×1440, 43:18 or 2.388:1
To compare, at a 5ft viewing distance, horizontal resolution for the 34" ultrawide would be 114 pixels per degree, for the 43" 4K tv 111 pixels per degree. Virtually the same, both at the upper limit of human vision.
So basically going from ultra wide to 4K you’re only expanding your vertical view, rendering more pixels for the same perceived resolution.
However you generally sit further away from a tv and closer to a monitor. Thus you can reduce the render resolution for the tv compared to the monitor, since you won’t see that extra detail anyway. Thus set the display resolution to 4K, but reduce the render scale the further away you sit.