Looking at your numbers i think that you could get the job
done with a cluster of 2 Domino servers with the HW specs you mentioned.
Put all 4000 mail files on both servers, assign server1 as the home server
for users 1...2000 and server2 as the home server for users 2001...4000.
So under normal circumstances a Domino server would have to handle not
more than 2000 active users. Only in case of failover the load on one server
would increase for a certain period of time.
If you look at the figures from the old Domino 8.5 Enterprise
Cluster Mail Performance tests
https://www-10.lotus.com/ldd/dominowiki.nsf/dx/domino-8.5-performance-for-lotus-notes-users-inotes-users-and-server-performance-for-enterprise-server-clusters?open
they already had 3000 active users per server and 6000
users in total on server hardware less powerful than what you have available.