Ever heard of Friday The 13th The Final Chapter?
Soon followed by "A New Beginning" and about another 5 after that.
Yeah but they're horror films which tend to suffer more from unneccesary sequelitis (although i have to say that slasher films getting loads of sequels bother me far less as it's generally entertaining to see annoyingly stupid teens getting carved up by Jason/Michael/Freddy).
In recent years the only comedy series to have countless sequels is the American Pie series, but four of those were direct to DVD and so not "proper" sequels, and best ignored anyway. I think this probably will be the end of The Hangover series, especially since the budget keeps skyrocketing with each installment ($35 million for Part 1, $80 million for Part 2, and over $100 million for Part 3 due to the stars being bigger than they were back in 2009). There might be a "The Hangover: Reunion" type film in 10 years or so like what happened with American Pie last year, but i doubt we'll be seeing Part 4 in the next two years.