if you are using 'Revelations' crack then you need to copy the M4_THOR_RD.DLL to the 'Bin' folder which is in the directory of where you installed the game.
If you are using 'Barrys' crack then you will need to replace the Myst.exe and the M4_THOR_RD.DLL files. Both files go into the Bin folder.
You are only backing up the original files in case a future update comes out which needs the original files to install. If you havent backed up the original files and the future patch needs them, then you will more than likely need to reinstall the game again.
And Im also guessing if you use Revelations crack then you will probably need a backup copy of the CD to play!
Whereas if you use Barrys crack then you more than likely wont need a CD to play! But that last bit is after all a guess?
EDIT: Ive just noticed from one of your earlier posts that you have the #2 crack. Revelations Crack! Download Barrys as well just in case!