HDD bays

Looks like my case is missing an HDD bay on the bottom. From what I understand, there are 4 3.5'' SATA drives I can connect but I can only see 2 in the back and one on the bottom, not two as it is shown in the manual. Where is the 4th one might be? Also, I have space for an additional 3.5'' bay on the bottom. Who can I contact to get the extra one?