Optimal SQL Server 2012 Disk Configuration
Best practises state that the optimal storage configuration is to have at least three volumes: One for the OS, another for the Database files and the third for transaction logs and to that you should add a fourth one for tempdb. 

Sometimes you just don't live in an optional world though. If you cant possibly get a third pair of disks, I would recommend that you benchmark different configuration using SQLIO. In your case you might be able to squeeze more I/O from the raid controller, without sacrificing redundancy by setting all four disks up as a single volume using RAID 1+0 and create four partitions on that for the os, data, log and tempdb. Only a benchmark will tell.