I have the N54L with 16GB. I tested it with 4 x 4TB drives in RAIDZ2 and the memory never maxed out.
For a 4 disk array in RAIDZ2 of 2TB drives, 8GB is more than enough to run ZFS and do scrubs. There are plenty of people with 16GB with way more drives. The manual may say use a gig per TB, but for your N40L and your 2TB drives you'll be fine.
The only area you may struggle with is deduping, which if you do not use won't be a problem for your CPU.