Why are some TV's £399 and other £3,999

What an earth can make it so different. (Obviously there is a quality improvement but so much money for a TV the same size)

I'm itching for a new TV and there seems to be a range of 55 inch models between £350 and £500 that are

55" Smart 4K Ultra HD HDR LED TV

Is that what most people get when they upgrade now?

The picture on my 8 / 9 year old Panasonic HD plasma is still excellent but I'd like a bigger screen