Something happens when retaining walls are more then 4ft tall. The amount of pressure on them increases to a point of needing some engineering. Less then 4ft, and it's all easy pleasy.
The two biggest issues are water and water freezing. If you can drain the water away from the side of the bock, you will eliminate all of your potential issues. Adding a deadman, or anchor, or some sort of reinforcing structure to the wall will help, but there is a limit to what it will do when fighting water, and water that freezes.
How much rain do you get in the worse case scenario?
Does it freeze there when the ground is saturated?