SQL help - concatenating primary index

How do I create a primary key that is the concatenation of two columns in a table that already exists without deleting either column? For example:

Code:

+-------------------+---------------+------+-----+---------+-------+
| Field             | Type          | Null | Key | Default | Extra |
+-------------------+---------------+------+-----+---------+-------+
| col1              | varchar(255)  | NO   |     |         |       |
| col2              | varchar(255)  | NO   |     |         |       |
| col3              | float         | YES  |     | NULL    |       |
| col4              | int(11)       | YES  | PRI | NULL    |       |

In this example, col4 is my primary key, but I have come across a situation where I need multiple col1 values associated with the same col4 value. The easy solution is to concatenate col4 and col1 as the primary key, but I can't rebuild the table from scratch because it is populated with a lot of entries.

Thanks,

 

nvm. Stupid question.

If anyone was curious, it was pretty simple:

Code:

ALTER TABLE table1 DROP PRIMARY KEY;
ALTER TABLE table1 ADD PRIMARY KEY (col4, col1);