Read the manual for your PERC 4. It's downloadable from Dell website.
From memory I think they do allow RAID5 capacity expansion w/o deleting existing array. Any expanded space will be added as unallocated partition which you'll have to allocate then format before you can store data on them.
One question. Who on earth got you to put OS on a RAID5 array together with your data? Even for hardware RAID5 with good write speed like your PERC 4, a server (especially database) gets hammered a lot, having OS and data on the same array is going to slow it down alot. Typically a server runs its OS+app on RAID10 with 4 small drives or multiples in RAID0. The data is on another array which has redundancy.