Hello,
The array allows you to extend the vdisk size by adding a drive and using the extend feature. There is no way through the firmware to decrease a vdisk size.
If you no longer want the vdisk to have all 4 disks, you will need to perform a full backup, verify your data, and then delete the vdisk. You can then recreate a new vdisk using only the 3 drives and restore data from backup.
Cheers,
Shawn