NAS full - How to expand capacity?

I have an EX4100 with 4x4TB drives in RAID 5 format giving me almost 12TB of usable space.
I have filled the 12TB with movies etc. and now need to create a plan to increase the total capacity.

My bad for miscalculating the size needed. (I still have about 500 movies to add to the NAS plus more moving forward)

I look forward to your suggestions.
Thanks

Hello,

please refer to the following KBA article: My Cloud RAID Online User Guide and Solutions