Index problem

Is there a way to force Informix Online DS (7.13) to use a particular index. Consider the query: select.... where a=1,b=2,c=3,d=4 I have a composite index on a,b and c I have another index on a,b and say, e - which isn't in the original query. Informix is using that second index and causing a major slow down of the system. I've tried dropping and recreating indexes and updating statistics but to no avail. Any help VERY GRATEFULLY received!!!!