informix does not use indexes?

OWS 7.22 on SCO UNIX: The following SELECT-statement in dbaccess takes a long time until I get a result , although I have declared an index on col4 (the table "table" has about 400000 rows). But when I use "set explain on" and look at the output of this statement, I see that there is no index used for the search, just SEQUENTIAL SCAN. I also tried to set optimization high or low. select * from table where col4 = 2 Are there any options to set so that informix uses indexes, maybe?