I'm a week behind on this apparently, but the first long trailer for Shrek Forever After due out in May 2010 is now available for viewing. I've embedded it below.
This trailer is more appealing to me personally than any of the ones I saw for the third installment. Hopefully it will also be better than that one! Supposedly this will be the last Shrek movie although a Puss in Boots spin-off is still on the docket.