according to WIKIPEDIA so i'm not sure if it's accurate but it says that the grudge 4 has been announced and some other director's replacing the guy from the 3rd film and it's supposed to be released sometime in 2011/2012 most likely 2012 i think.
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I read that article too. I don't know what to believe in that article. It says that posters and footage were released at Comic Con 2010, yet, I tried looking for these posters and more information on the movie but none existed. It also says that Sarah Michelle Geller and Ryan Reynolds are suppose to have roles in this film but no news was ever heard of.
I'm pretty sure there is a Grudge 4 coming our way within the next few years but so far, it hasn't been 100% confirmed.