They will probably try and see what they could do for future movies that would be popular.. Would they make even more with just gruesome deaths, or will they have another one to answer all the questions and finally get to the mysteries behind it all..
I think it probably will happen, I think it'd be sucessful if they answer questions raised in previous films, but I can't really say I'd not watch a fourth just because it may not do that.. If you're a fan of all 3 so far, you'd probably be a fan of anymore to come.
But i'm looking forward more to Jurassic Park 4 and Scary Movie 4, each of these also seem to have had similar storylines in previous films but they are all different enough to be interesting and worth the cinema ticket.
But to answer your question, I don't think there is any news on if a 4th has been planned. If there is, we may not find out till the end of this year and then it may not be released for a few months after that.. I think the 3rd has come around 2 years since the last one.