I've checked the old page for Overture 4 and it's listed that the program will work on OS X 10.4.0 or higher, but I won rely on that for El Capitan compatibility and most likely it won't work, but you can apply a patch if it doesn't starts normally.
Secondly, there is Overture 5 released which supports latest versions of OSX as well: https://sonicscores.com/downloads/
