In 2010, Gary Marsh, president of Disney Channel confirmed that there will be a fourth installment in the series saying, "I’m excited to be part of a franchise that, thematically, encourages kids to believe in themselves. Like the first three movies, my goal is to keep the art form of the traditional musical alive, yet bring a contemporary emotional response to the story and its characters through song and dance."
It was also confirmed that the main stars such as Zac Efron, Vanessa Hudgens, Ashley Tisdale or any of the original Wildcats gang would not be returning as the lead roles but entirely different actors will be cast for the film. Reports said that the fourth film would have the title High School Musical: East Meets West and would be directed by Jeffrey Hornaday. It was also reported that the plot would follow around a love triangle set against the cross-town school rivalry between the East High Wildcats and West High Knights.  However, the movie was never shot and never made it to the big screens.
In an interview with Showbiz Spy in March 2013, Efron stated that he would be open to filming a fourth film, saying, "I can't say for sure if it will happen but deep down in my heart, I would jump at the chance." He then went on to say that he was "pretty sure [the rest of the cast] would feel the exact same way." In December 2013, it was confirmed that the fourth installment would be made to be released in March 2015.
There had been announced a High School Musical 4 called the wedding where Troy and Gabriella get married. Corbin Bleu, who played Chad in the franchise.
Very unfortunately, this was all a joke. Looking at websites fans of the High School Musical franchise got very angry that this was a joke and said High School Musical should not be joked about. Also that it "shattered their dreams."
High School Musical: East Meets West was not released in March of 2015, neither was it ever mentioned to be released during this time.
As of May 2015, there has been no confirmation of a fourth installment even though most of the franchise's main characters have stated in interviews that they would be interested in another film.
Apparently, the reason behind the 4th installment being delayed for so long was due to Zac Efron dealing with substance abuse. He is rumored to finally be clean and better. In a recent article on radaronline.com, it discusses the fact that Zac Efron is finally ready to reprise his role in a 4th film. It also discusses the fact that Kenny Ortega only wants a 4th film to happen if Zac Efron and Vanessa Hudgens reprised their lead roles once again.
As of June 15 2015, There is a rumor on Twitter that High School Musical 4 will happen and start filming this fall, Becky G was not sure if it was a hoax or real. On Twitter there is a paper of the cast roles of dancing and singing cast members and the producers saying that they are not sure if they will keep the name or have a different one .
On January 20, 2016 the Disney Channel celebrated the 10th Anniversary of High School Musical by airing the first High School Musical movie. During the commercials, some of the characters (Corbin Bleu, Ashley Tisdale, Vanessa Hudgens, Monique Coleman, and Lucas Grabeel) discussed the 10th Anniversary. Zac Efron was unable to be with them but Disney aired a prerecorded video of Zac Efron for the 10th Anniversary. Unfortunately, the characters did not address any possibilities of a 4th film.
On March 1, 2016, Disney announced an open casting call for a fourth installment. The movie will follow both the East High Wildcats and the West High Knights. It will be directed and choreographed by Jeffrey Hornaday and written by Dan Berendsen and Peter Barsocchini. As of now, there is no confirmation of original cast members returning.  
If you're between the ages of 14-17, you can audition here. They will be accepting auditions April 18th through May 2nd. If you would like to audition, you better be ready.
The Couples of High School Musical 3: Senior Year