Following is my experience.

SQL> explain plan for
2 update prtcpnt
3 set f9= 1
4 WHERE f3 = :b1
5 AND f4 = :b2
6 AND f2 != :b3
7 /

Explained.

SQL> @showplan
SQL> set echo off

Query Plan COST
-------------------------------------------------------------------------------- ----------
UPDATE STATEMENT [CHOOSE] Cost = 27 27
UPDATE PRTCPNT
INDEX RANGE SCAN PR_TEST 2

SQL>

After that I am analyzing Index, and then Plan shows cost more. What is the reason.

SQL> analyze index pr_test compute statistics;

Index analyzed.

SQL> @expl
SQL> truncate table plan_table;

Table truncated.

SQL> explain plan for
2 update prtcpnt
3 set f9= 1
4 WHERE f3 = :b1
5 AND f4 = :b2
6 AND f2 != :b3
7 /

Explained.

SQL> @showplan
SQL> set echo off

Query Plan COST
-------------------------------------------------------------------------------- ----------
UPDATE STATEMENT [CHOOSE] Cost = 445 445
UPDATE PRTCPNT
INDEX RANGE SCAN PR_TEST [ANALYZED] 56

SQL>

The table contains more than 500000 records.