Need advises for a new MySQL architecture
This sounds like a good use case for Galera.
http://codership.com/content/using-galera-cluster

You could also use something like this:
Each server is a master for it's copy of the user table (I assume the user table is sharded?). Then there could be a central server with MariaDB or MySQL + Tungsten Replicator which is a slave of all the other servers.

And then you could create another master (extra instance on one of the servers?) which hosts the master copy of the content tables.

CM = Content Master
U  = User Master
UC = User Combined

Server 1: CM + UC (2 Instances)
Server 2-5: U

For the content tables (1 master, 4 slaves):
CM -> U1
   -> U2
   -> U3
   -> U4

For the user tables (4 masters, 1 fan-in slave):
U1 -> UC
U2 ->
U3 ->
U4 ->

Galera and MySQL Cluster provide synchronous cluster where you can write to any server. MySQL Cluster needs 2 data nodes and 2 SQL nodes as a minimal setup (the mgmt node can be combined with a SQL node). MySQL Cluster does not use InnoDB or MyISAM. Galera needs 3 nodes and does support InnoDB.

Investigate Galera, Tungsten (and MySQL Cluster aka NDB?).

Read MySQL High Availability (O'Reilly).