

4K = 3840 x 2160 pixels = 8,294,400 pixels per frame
Assuming 8 bits of color depth (lowest common depth) = 66,355,200 bits per frame (66MB)
At 24fps (again, low end as mentioned by another commenter) = 1,592,524,800 bits per second (1.5GB)
95.5GB per minute of footage per camera, assuming literally no compression.
95.5GB × 40 = 3.8TB × 3 cameras = 11.4TB
Raw uncompressed video takes up a LOT of space.
(Yes I used 1000 instead of 1024 for convenience)
Thanks, I was too lazy to explain that haha