I've had no luck searching how to setup writeback cache on a RAID10 setup using mdadm software RAID. I'm trying to set this up for a 4x256Gb Samsung 830 SSD setup which only has 400Mb/s write speed... CentOS6
If you are using those 4 to cache for a large rotational disk (or other slow disk), look into dm-cache, bcache, and friends. They're not built into mdadm but can be used on top of raid devices.
If you're only able to write 400MB/sec to those four SSDs, you should check what the bottleneck is; I doubt it's io speed, unless you're using a crappy controller. Check your iowait%, if that is high, try moving one or more of the disks to a different controller (like a PCIe sata adapter). If it is low and the cpu usage is 100%, your raid might be CPU-bound.
Most common filesystems will automatically detect optimal tuning parameters of linux softraid when you create them on md devices.