compressor 4 os 10.8
Will compressor 4.2.x run on OS 10.8.4?
Mac Pro, OS X Mountain Lion (10.8.4)
Sorry, but it needs at least Yosemite. Compressor 4.2.1 - Technical Specifications
If you can't upgrade the OS for whatever reason, you might consider creating a bootable volume – either from partitioning your current system drive or on a separate drive – and installing 10.11 (and Compressor) on that.
Russ