adding drive to raid 5

Let me try to get this right. If we have a server with 3, 9 gig SCSI drives set up on a raid 5 array and we add a fourth drive in can the fouth drive become part of the first array or is it a second logical array. If it is a seperate array (wich is what I think happens)is the information on the fourth drive backed up on the first array as though it was part of the array? I hope this makes sense to someone.