Master-master database replication on 5 servers
I'm just throwing this out there but have you thought of using a MQ for writes?

We're using RabbitMQ for writes and federating it out to multiple data centers where each one has a flexihash cluster of ~20 Redis servers. So far it's working well for us and you can increase or decrease the size of each cluster independently as long as your rebalancing them appropriately. Hope this helps.