please help

hi, i recently downloaded flight simulator for the pc. it comes with 5 files, cd1, cd2, cd3, cd4 and "no cd crack" file. i have copied the first four files onto cd's and installed the application. when installed it says insert cd4 and restart, i have done this many times but the same message appears and it doesnt let me play the game. i'm not sure what the "no cd crack" file is or how to incorporate that into the game. will someone please help me.