How to optionally add disks

I need to add up to 4 disks of different sizes.  I tried setting the count to ${input.optionalDisk1Size == 0? 0 : 1} but that stops me from adding any other disks to the blueprint.  I can't seem to figure out how to add UP to 4 disks where the sizes vary.  It seems that I need 4 separate disks on the blueprint that may or may not be provisioned.

Any suggestions gratefully accepted.

Carl L.