MySQL fine tweaking for performance

Hi, I Need to tweak a production MySQL database. There are no slow queries, all searching fields are indexed, etc, etc. But... These servers run on a dedicated Ubuntu Virtual Machines with about 12 GB or ram, and can use up to 4 cores. and are only using about 3GB, so I need to improve the caching, the only service running on the VM is the MySQL database. I can give you the current configuration files and any info you want, but cannot give you access to the servers in any way. You'll need to provide me conf files to optimize the performance with the current resources. The server runs reading 98% and writing 2% The DB needs to serve a pool of 20 Rails unicorn workers. Please, bid only if you know what we are talking about, and been there done that. Thank you,