Reason 4 is software by Propellerhead that allows users to create music on their home computers. While Reason 4 is compatible with every Mac OS greater than 10.4, the software might not mount properly on your Mac if you download it from the official website instead of purchase the DVD. If that is the case, Disk Utility is the solution. It is a utility program that comes with every Mac that mounts disks with ease.
Open the Application's folder and open "Utilities." Click on "Disk Utility."
Select the "Reason4.dmg" file from the files on the left-hand side of the Disk Utility window.
Select "Mount" from the tools menu at the top of the window.