SS is not appropriate here. If you want to change '1234' in positions 5-8 to '6789' in positions 5-8, you would use:
SS is for finding a string anywhere in a record.
FINDREP is for finding and replacing a string anywhere in a record.
Finding and replacing a string at specific positions does not require either.
If that's not what you want, then please explain more clearly what you do want. Show an example of your input records and what you expect for output. Explain the rules for getting from input to output. Give the RECFM and LRECL of your input file, and the starting position, length and format of each relevant field.