Plans for a fifth were scrapped. This is the last one, besides the spinoff.
When this movie makes a few hundred million again, they'll be more than happy to go onto the next one. 5 has officially been a go since before part 3 even came out. And again, absolutley no one will remember in three years that this one was advertised as the last.
Shrek 1 made nearly 500 mill worldwide, part 2 made 900 mill, and 3 made 800 mill, and thats not even counting the dvd sales, merchandise tie ins, or the ratings on the annual repeat of the new Christmas special.
Unless part 4 is absolutely abysmal and performs incredibly badly for some reason (and given the sucess of Shrek 3, Ice Age 3, Pirates 3, and Spiderman 3, quality is obviously not a factor compared to name recognition) and makes like, 5 bucks…
Quality doesn't matter when there's that much sheer profit to be made.
There will be a Shrek 5 in 2013. They're just pretending there won't be right now so that 4 does better.
Hell, Kung Fu Panda is slated to have like 6 films and its only 1 movie in.