How to properly and correctly install Reason 4 without errors:

1. Ctrl+click (or right click) on the DMG File, and click "Get Info". (or simply click CMD+I)
2. Check the "Locked" box near the center of the window.
3. Mount "Reason_4.dmg" using Roxio Toast 6 Titanium, you can download it from here.
(In Tast 6 Titanium, the procedure is: "Utilities" > "Mount Disk Image", or simply right click the "Reason_4.dmg" file and choose "open with" > browse for "Toast 6 Titanium" in your Applications Folder)
4. Open the newly mounted "Reason 4" on your desktop and proceed with the installation as usual, (usually it's just a "drag-'n-drop" operation) and use the serial in the description above.

(Do NOT copy the sound banks manually to the Reasons Applications folder, but instead, once the main Reason files/folders are copied to the Applications folder, start the "Reason.app" from your Applications folder and let Reason autocopy/autoconfigure everything for you, incl. copying the 2 main sound banks to the main Reason application folder. During this initial setup your admin password will be needed, you'll need to insert the serial provided (use any name in the Name field) and the app will be registered automatically.)

5. Once the initial v4.0 is installed and the app has opened once, proceed with the same procedure for the "Reason_4.0.1.dmg" update, also using Roxio Toast 6 Titanium to mount the dmg file.
6. Once the v4.0.1 update is installed, proceed with the same procedure for the "Reason_Refill_Packer.dmg" update, also using Roxio Toast 6 Titanium to mount the dmg file.
7. Once the 3rd dmg is mounted and installed, you can now close Roxio Toast 6 Titanium and unmount the 3 dmg files as per the regular procedure. (Rightclick on the dmg file and choose "unmount")
8. You're now ready to use Propellerheads Reason v4.0.1. Enjoy!