Plasmas do NOT "burn out" in 4 years. Their estimated lifespans are actually pretty similar to standard TV sets. There are also plenty of plasmas with built in speakers - MOST people who buy a $4000 TV already have a $$$ home theater setup, so we'd actually prefer the display not to have cheap speakers built in, FYI.
You can buy an HDTV plasma, but not all plasmas are HDTV.
The picture quality is up in the air right now - a lot of the cheap plasmas do not look as good as a standard HDTV set. However, the improvments over the last few years have been dramatic, and $5000 will get you a pretty sweet looking plasma display.
Their biggest competition is LCD technology, which is getting bigger and better as well.
IMO, you just can't beat a plasma for the coolness factor...