Thank you for joining the Rapid Storage Technology Community. I am sorry to hear that one of your disks broke.
I would like to let you know that the RAID volume cannot be reduced to 3 drives; however, I have two suggestions that could help in this scenario. The first one is to change the broken drive and rebuild the volume, and the second option is to back up the information and create the RAID 5 with the 3 remaining drives from scratch.
Hope this information helps.