If it's going in a full rack then why does it need to be silent? It's not like 4x raptors are going to be that quiet anyway, so the stock fan or similar would be fine.
If the rack is not full, then get a 4U case and use a bigger cooler and fans.
Your specs seem very overkill for "Email Archiving". What software are you going to be running that requires so much power and fast disks?
I dont have the 4u space. I have 2u space. I dont want to waste space where I dont need to. My customs build server needs to be future proof. The Email archiving program needs io for running searches, etc.