3-4 is probably a very conservative estimate. Realms are most certainly clusters of servers, though they almost certainly are not the rack mount Dell kit you'd look at on their website.

I'd guess they have a mixture of databases for things like auction house transactions, bank contents, persistent character data storage, mailbox functions, etc. Then the live world likely runs on a cluster of machines which handle mob behavior, spawning, resource spawning, arbitrating combat, etc. Its pretty well understood from observation that instances seem to run independently of both Azeroth and Outland which also pretty clearly run independently of each other.

Whether the servers load balance population based on zone location within the larger grouping of Azeroth and Outland is anyones guess, and which zones are grouped together is likely based more on average populations than "geographical proximity" on the map.

The key to running a server farm the size of Blizzards and keeping the most people the most happy the largest percentage of the time is spreading the load evenly.

This brings an interesting question up for those realms currently experiencing queues, namely would logging out in a low population area such as Desolace net you a shorter queue or is population for that controlled on a realm level?