According to Sci-Fi Wire Shrek 4 has a release date already.

May 10, 2010

No word yet on the plot of the 4th Shrek film, but we do know it won’t be the last.  A 5th film is also in the works.  There is also no words on if production is continuing on the previously mentioned direct to video Puss-N-Boots movie, but I’m sure we’ll know more about that soon.

If you’re a Shrek fan, you will have a decade or so of new Shrek content to look forward to.