Hi,
Thank you for your query.
As you do not have any delimiter, you can split.
Because the length is fixed, you can use MID function in SSRS which is equivalent to SUBSTRING.
Here us the example expression:
=MID(Fields!StringData.Value, 1, 4) & " "
& MID(Fields!StringData.Value, 5, 4) & " "
& MID(Fields!StringData.Value, 9, 4) & " "
& MID(Fields!StringData.Value, 13, 4) & " "
& MID(Fields!StringData.Value, 17, 4)
This shows following output:

You can also use the same expression to show this in separate columns:

If you are using this value multiple times in the report, it is better to do it in the SQL query:
SELECT CONCAT( SUBSTRING('ABCD1234ABCD1234ABCD', 1, 4), ' ',
SUBSTRING('ABCD1234ABCD1234ABCD', 5, 4), ' ',
SUBSTRING('ABCD1234ABCD1234ABCD', 9, 4), ' ',
SUBSTRING('ABCD1234ABCD1234ABCD', 13, 4), ' ',
SUBSTRING('ABCD1234ABCD1234ABCD', 17, 4)
) AS FormattedValue