Split the results of a query into parts
To solve your issue, I did the following:

Create table and data:

CREATE TABLE salesperson (id INTEGER, salary INTEGER);

INSERT INTO salesperson VALUES (1, 100000);
INSERT INTO salesperson VALUES (2,  80000);
INSERT INTO salesperson VALUES (3,  60000);
INSERT INTO salesperson VALUES (4,  50000);
INSERT INTO salesperson VALUES (5,  30000);
INSERT INTO salesperson VALUES (6,  25000);
INSERT INTO salesperson VALUES (7,  12000);
INSERT INTO salesperson VALUES (8,  10000);


Then run this query (SQLFiddle here):

WITH cte AS
(
  SELECT id, salary, NTILE(4) OVER (ORDER BY salary) AS quartile
  FROM salesperson
)   --     &lt;&lt;== you can do a SELECT * FROM cte to see what's happening!
SELECT
       id,
       salary,
       CASE quartile
         WHEN 1 THEN salary * 1.08
         WHEN 2 THEN salary * 1.08
         WHEN 3 THEN salary * 1.06 
         WHEN 4 THEN salary * 1.04
       END as new_salary
FROM cte;


Result:

ID  SALARY  NEW_SALARY
1   100000  104000
2   80000   83200
3   60000   63600
4   50000   53000
5   30000   32400
6   25000   27000
7   12000   12960
8   10000   10800 


HTH and welcome to the forum! :-)

You can read the Oracle NTILE documentation here and an explanation from the excellent oracle-base site here.

EDIT:

You can also do it in one pass - but I think the cte (COMMON TABLE EXPRESSION - or WITH clause) shows the thought process more clearly. Thus:

SELECT
       id,
       salary,
       CASE NTILE(4) OVER (ORDER BY salary) 
         WHEN 1 THEN salary * 1.08
         WHEN 2 THEN salary * 1.08
         WHEN 3 THEN salary * 1.06 
         WHEN 4 THEN salary * 1.04
       END as new_salary
FROM salesperson
ORDER BY id;


Same result (fiddle)!