Warning: the following content has been encrypted with an exceptionally bad password. Proceed with caution.
Step 1: Crack the 4-digit code using only your wits and a pen and paper.
Click here for Step 2: Use a flowchart to narrow down the possibilities.
And for a deeper analysis, try Algorithmic Forensic Analysis.
Or, if all else fails, just give up and try the algorithmic throwdown.