Don't blame them for wanting to make another one, its a successful franchise which always makes good money at the box office. The films are crap (imo) but it has a huge fan base. From a business point of view if you could sell your household garbage you would, hollywood is not concerned with making good movies... just good money
spoof movies have an endless supply of source material, and that's why you always get many sequels, and they're also very popular at the cinemas, so hollywood has no reason why not to make sequels
Glad to see that both Anna Faris and Regina Hall(Brenda who died in Scary Movie 3) are back for the 4th film all that will be missing is the wayan brothers, the third film had nothing on the first two.