Best big flat screen under 4k

If you are looking for Plasma TV Buying Guide, go to PlasmaTVBuyingGuide.com. I have a Panasonic 42" plasma. Since all have similar features, I chose Panasonic over other brands primarily for 5 reasons.
1. Brightness ratio -- 4000:1. Mine was last year model w/ ration of 3000:1, which at the time was the leader in its class.
2. Real Black Drive System. Still the leader in its class.
3. Based on the breakdown testing comparisons of Panasonic and others on PlasmaTVBuyingGuide.com
4. Makes its own glass.
5. Many generations in making plasma. I believe it is currently the 6th generation.

To summarize the answer for your question, 37" Panasonic plasma should meet your $4K budget requirement and your screen size requirement of 42"+/- few inches, if you are in a hurry of getting read for upcoming super bowl.

I would stay clear of the Gateway Plasma's for now, the ones I've seen look washed out, and they have consistently gotten horrible reviews. I agree with the Panasonics, though they are outside of this price range. If you must go plasma, you might consider saving up the extra $ and going with the Pani.

Since you specified "flat screen" models, rather than "plasma" or "LCD" models, I will assume that you are including flat screen direct-view (CRT) TV's. One of the best is the 40" Sony Wega XBR HDTV. It has a superb picture, although the set is pretty heavy (around 200 pounds, as I recall).

Given that my 36" XBR clocks in at around 250 lbs, the 40" has to be a monster. The problem isn't just weight, its unwieldy to move since there isn't anything to grab. You might think that ain't a big deal, but how many times do you rearrange your gear? Then again, it is perhaps the greatest expression of a tube based TV ever made...

After having thrown in my $0.02 based on thinking you were specifically looking at plasmas, I'll note I really am impressed by the Samsung DLPs. Just read a great thread on AVSForum on them, and apparently Samsung demoed a 1080p (yes, 1080*P*) 1920x1080 61" DLP at CES... 5000:1 contrast ratio... This, in addition to a series of new sets to be launched shortly with the xHD2+ chip that gets you to a 2500:1 contrast ratio... Might be worth waiting a bit.

I am definetly with Edesilva in the opinion of the regular CRT Tvs being way too cumbersome to move around, lest they are small in size (19".) The Samsung DLP have also caught my eyes, but I have read elsewhere about the issues that many of the competing new formats have....Plasma, LCD, DLP, back and front projections....Black level problems, burn in, bulb replacement costs etc....all in no particular order. If I consider spending around 3-4K on a next generation TV set, then, shouldn't I be expecting a long trouble free enjoyment? I had regular CRT tvs that lasted and still are lasting a good 10+ years. I wonder if these new sets will last that long, reliability wise.