mysql> CREATE TABLE tt (
    ->   a TEXT, b TEXT, c TEXT, d TEXT
    -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8;
ALTER TABLE tt add UNIQUE key (a(6),b(6),c(6),d(6));
insert into tt set a='banana',b='banana',c='banana',d='banana';
insert into tt set a='banana',b='banana',c='banana',d='banana';Query OK, 0 rows affected (0.00 sec)

mysql> ALTER TABLE tt add UNIQUE key (a(6),b(6),c(6),d(6));
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into tt set a='banana',b='banana',c='banana',d='banana';
Query OK, 1 row affected (0.00 sec)

mysql> insert into tt set a='banana',b='banana',c='banana',d='banana';
ERROR 1062 (23000): Duplicate entry 'banana-banana-banana-banana' for key 1