How can I add more than four disk images to qemu?

Qemu fails when I try to add more than 4 disk images using -drive file=...,index={0..3},media=disk,format=raw. The error I get for each additional drive is similar to the following:

qemu-system: -drive ....: machine type does not support if=ide,bus=2,unit=X

How do get qemu to accept additional drives?