Add 4th HDD to RAID 5?

[Pilbromatic]

Hi all,
I've recently put 3x 4TB drives (WD Red) in my ix4-300d and I've got them set up in RAID 5. I'm wanting to add a 4th drive to the array - will this just work or will the whole array have to rebuild and wipe all of my data? I still have the data backed up just in case but if anyone's tried it I'd be interested to know - I've searched and searched online but haven't found an answer! Thanks!