Do not want concurrent request set to end if parameter is missing

We want to set up a scheduled concurrent request set containing four steps, each step runs the same concurrent program but with a different input parameter.  The input parameter is a file, so for each step a different file will be processed.  We want all four steps to run even if one of the files (input parameter) is missing.  For example if step one has input parameter of FILE1, step two has input parameter of FILE2, step three has input parameter of FILE3 and step four has input parameter of FILE4.  If FILE3 does not exist when the concurrent request