I have to say that this film is a pretty good example of a studio making one too many films in a series. The first film was pretty good and it was followed by an excellent sequel. By the third film it was becoming obvious that the studio was running short on ideas and this film can be seen as a pure cash-in.
This is not a bad film, its just that the story is not that up to par. This film is nearly devoid of the fun pop culture references that made the series so far and it is now features a fairly generic plot with a slightly mediocre villain (who doesn’t even have a celebrity voice).
As a result, I have to say that the Shrek series ends with a bit of a whimper rather than a bang.