How to insert columns with adding sequence column?

In an Oracle database (11gR2), I have a table my_table with columns (sequence, col1, col2, col3). I want to insert values into the table that are queried from other tables, i.e. insert into my_table select <query from other tables>. The problem is that the primary key is the four columns, hence I need to add a sequence starting from 0 up till the count of the rows to be inserted (order is not a problem).

I tried using a loop like this:

DECLARE
    j    NUMBER;
    r_count number;
BEGIN
    select count(1) into r_count from <my query to be inserted>;
    FOR j IN 0 .. r_count
    LOOP
        INSERT INTO   my_table
              select <my query, incorporating r_count as sequence column> ;
    END LOOP;
END;

But it didn't work, actually looped r_count times trying to insert the entire rows every time, as logically it shall do. How can I achieve the expected goal and insert rows with adding a sequence column?