It all boils down to what your after, I recently went along the widescreen path and spent countless hours reading, research and finally narrowed it down to 3 makes, Dell, Samsung or Viewsonic.
Dell at present has some serious quality issue's with their panels at present so they were quickly scrubbed off the list, Samsung whilst a very nice screen you pay through the nose for it and the little extra's you get.
I ended up going for the Viewsonic widescreen, whilst it's only 20.3" it was very cheap and Viewsonic have a good name for quality products plus an awesome warranty which can't be said for all of the others at the time of writing this. Viewsonic is the only company to the best of knowledge which will replace the screen if even 1 pixel is dead whereas others and lets pick BenQ as a sucker you need to have 4 dead black pixels, 4 white or 10 sub pixels in order for them to exchange it. I think it's safe to safe once you find 1 it will drive you completely mad. On the otherhand we have Dell who think it's quite OK to have 6 dead pixels before exchanging it.
Anyway enough about warranty rubbish and lets get back to it, the Viewsonic was half the price of anything else out there. The Samsung for example for an extra inch was near enough to $400 more which when it boiled down to it I couldn't justify it plus the Viewsonic looked nicer to my eye's which is very important (you don't buy a car without looking at it right?).
What on earth do you need 4ms for? As you know I spent 8.5 hours a day at work at a computer and then some more countless hours when I get home and found 8ms to be perfect. IMHO these 'ms' figures are a crock, it's a manufactures way of saying our product is better than the others and they are getting away with it. Watch the way they word the response time!
As far I know a 21" 4ms I think your going to have a tough time finding anything in that range. 19" is the way of going if you so require the 4ms.
There's my quick view of it
Don't mind the mistakes I'm off to have tea now.