Fan-In replication 4 Master -> 1 slave
There is no solution for fan-in with traditional MySQL replication. 

Another option is Tungsten Replicator. Here are a couple of posts on how it supports fan-in replication:


http://narmitag.wordpress.com/2012/09/27/setting-up-fan-in-using-tungsten-replicator/
https://code.google.com/p/tungsten-replicator/wiki/TRCMultiMasterInstallation


Another option is Galera replication, which is integrated with Percona XtraDB Cluster and MariaDB Galera Cluster. In Galera, all cluster nodes take replicated changes from all other cluster nodes. Which satisfies your goal of fan-in -- sort of -- but it means that all nodes have all the data.

For what it's worth, MariaDB 10 made their General Availability announcement on March 31, 2014.

I don't recommend the Federated storage engine. It should really be deprecated, because it's slow, buggy, and fragile. My feel is that MySQL is considering scrapping it. See http://www.tocker.ca/2013/10/17/how-do-you-use-the-federated-storage-engine.html