Can I use only 1 bay on a Pro Series?


I have a MyBook Live that has decided to no longer working and I am trying to figure out what to replace it with. I know that i could go with a MyCloud, but the Pro Series with the multiple bays is intriguing to me. I am wondering if I could buy the 4 bay and add drives to it as I can afford it or as needed. For instance i would buy the Pro4100 and a 8TB Red NAS Drive now, then down the road add another Red NAS Drive. Is this something that can be done?