O.K. I'm going to try and take a stab at walking through the decryption process so its a little clearer for people who are unfamiliar with Command Prompt.
After you download the mi4 code program, you need to extract it to C:\mi4code\.
Then you need to copy the original firmware you downloaded earlier to same directory.
Next you need to open command prompt and navigate to C:\mi4code\ (you can do this by typing "cd C:\mi4code\" without the quotations).
In the command prompt window then enter "mi4code decrypt -s xxxxx.mi4 OF.bin" (where xxxxx.mi4 is the name of the firmware file (i.e. PP5022.mi4), this will decrypt the firmware and produce a file named OF.bin
The rest is self-explainatory.