When I've specified timber walls of that height, I've cribbed them. Run timbers perpendicular to the face of the wall, back into the slope, and tie them together with some timbers. For a wall 4 ties high, I'd probably crib the 2nd and 3rd course, staggering the joints like masonry. Actually, the stretcher-header-stretcher layout is Flemish Bond.
This gives you a massive wall that won't tip over. Generally, a depth of 2/3 the height of the wall is enough, but that can be varied some for ease of construction. I like them vertical, but a battered wall looks good to some people. Or you can go hog wild, step them back with exposed dirt on the steps to plant stuff in. You definitely need to crib the latter.
The ties are held together with spikes or rebar driven into drilled holes, usually the latter as you can get rebar anywhere.
I like to place some geotextile or landscape fabric against the back of the wall, and lay in some free-draining gravel to keep the water from behind the wall. For a big wall, or a significant back slope, I'll often lay a 4 inch perf drain along the base, and try to drain the water off somewhere. Otherwise, leave the bottom joints open a bit to act as weeps.