RAID-5 is a protection scheme where both data and parity information are striped across three or more drives. The parity information, which is used to rebuild data during a single drive failure, is spread across all the drives in the array in a stripe just like the data itself is. The total space consumed by the parity information is equivalent to an entire drive. So, in your four hard disk configuration, the fourth disk is not a spare but rather is the "parity drive."
However, be aware that this is not an actual physical drive but rather the space that would have been taken up by a physical drive if the parity information were not spread across all four drives. In short, in your RAID-5 configuration, all four drives are fully active, but from the operating system's perspective, total capacity is that of three drives.
This was first published in May 2007