Do you mean 4 x IDE drives i.e. with ribbon cables?
You could put your existing drive as Master on the far end of ribbon 1 (use little pin jumpers on rear of drives). And the Optical drive as Slave on the middle of ribbon 1.
Then buy another Hard drive and put it jumpered as Master on the far end of Ribbon 2.
SATA drives are only a little dearer than IDE(PATA) Drives and have faster data tranfer rates. One of these might be a better alternative, if your Motherboard supports SATA.