It's been my experience that modern LCD LED TV's will easily pass 100,000 hours (over 8 years).
Some brands are prone to failure (all this stuff is made in China basically). I don't think you need a warranty, but peace of mind having one can't be discounted. I usually get one if it isn't too expensive.
I like Vizio - personally. I have bought a 42 inch, a 50" and an 80". So far, no problems. My 50" is a great bedroom TV and my 80 is great in the living room. Personally, I couldn't see going lower than 70" if you have a big room for a lot of people.
As for 4K TV, they are basically the standard now till the industry tries to force us into 8K or whatever. Most equipment is running 1080p so there's no rush. In 2 years 4K will be even cheaper.