Here's the plot, according to the Acting Info blog:
High School Mucical 4 will revolve around "a classic love triangle" set against a new generation of East High Wildcats and their crosstown rivals, the West High Knights.We think it has potential. West High hasn't really been featured.
The fourth movie will star Jemma McKenzie Brown and Matt Prokop. Although they haven't been officially confirmed, Justin Martin, Bart Johnson, and Alyson Reed are expected to return.