SQL & PL/SQL :: Data Should Be Updated Back To Table

The requirement is, the combination of col1,col2,col3 and col4 should always be unique, and wherever the col1, col2,col3 are same then col4 should be the sequence, starting from 1. Likewise the data should be updated back to the table.I'm able to do this using PL/SQL. Can I do the same using a single update statement?

create table tab1 (col1 number(5), col2 number(5), col3 number(5), col4 number(5));

Existing Data:

insert into tab1 values (101,521,3,1);
insert into tab1 values (101,521,3,1);
insert into tab1 values (101,522,3,2);
insert into tab1 values (101,522,3,2);
insert into tab1 values (101,523,3,1);
insert into tab1 values (101,523,3,2);
[code]....