Please confirm "The value comes out as 2017.000000" is incorrect, and the result is actually 2021.000000 as shown below.

Also please confirm that DT_GRD_9_ENTRY is a VARCHAR column, not a DATE column.

If neither is true, please disregard this answer :)

CREATE TABLE sr_stu_demo ( DT_GRD_9_ENTRY VARCHAR ( 100 ), dob VARCHAR ( 100 ) );
INSERT sr_stu_demo VALUES ( '20170725', '1999' );
COMMIT;

SELECT (if length(sr_stu_demo.DT_GRD_9_ENTRY) = 8 
        then ((substr(sr_stu_demo.DT_GRD_9_ENTRY,1,4))+4) 
        else sr_stu_demo.dob endif) as cohort 
  FROM sr_stu_demo;

cohort
2021.000000

When you add an integer to a string, as in ((substr(sr_stu_demo.DT_GRD_9_ENTRY,1,4))+4), the string is implicitly converted to a number before the addition takes place.

If you don't take control over that implicit conversion, you may get a data type you don't want... in this case NUMERIC ( 30, 6 ).

You can use the EXPRTYPE function to determine what the data type is going to be:

SELECT EXPRTYPE ( 'SELECT ((substr(sr_stu_demo.DT_GRD_9_ENTRY,1,4))+4) FROM sr_stu_demo', 1 );

numeric(30,6)

How you "take control" is up to you; here is one example using CAST ( expression AS INTEGER ):

SELECT CAST ( (if length(sr_stu_demo.DT_GRD_9_ENTRY) = 8 
              then ((substr(sr_stu_demo.DT_GRD_9_ENTRY,1,4))+4) 
              else sr_stu_demo.dob endif) AS INTEGER ) as cohort 
  FROM sr_stu_demo;

cohort
2021