The file format M4a is a music file that's been protected by iTunes. It uses the MPEG-4 compression, but doesn't include video, and is audio only. If you want to compress M4a, you can do so by using native tools available inside of the Windows operating system. Compressing files makes them smaller so you can easily transport them from one computer to another or store them.
Click "Start" in Windows and select "My Music" or another folder where your M4a file is located.
Right-click on the file. Select "Send to."
Click the "compressed (zipped) folder" option. A ZIP folder appears, with the M4a compressed inside of it.