Supplementary files are not necessarily accepted for all journal types, and should be an optional component of the submission process. Additionally, Journal Managers should be able to configure how many supplementary files the system will accept per submission, and in what file types/sizes. (That last bit actually overlaps Bug 5607.)

There should be a Supplementary File configuration section in Journal Setup Step 3. Journal Managers should be able to en/disable supplementary file uploads, and to specify a maximum limit for supplementary files on submission. 

The default configuration should be for supplementary files to be enabled, and for the max set to 0 (unlimited). 

If supplmentary files are disabled, that particular submission step should of course be removed, reducing the number of submission steps to 4.