I'm afraid that won't work, unless the 4th disk is configured as a Hot Spare disk. In addition to this I don't recommend a RAID5 with 3 disks as this is one of the slowest RAID configurations you can have, due to the way RAID5 works.
You can certainly pull a disk from a RAID5 set, but this would leave the RAID in an unprotected state, i.e. if another disk failed you'd loose your data.
André