Strength Of I-beams...
You need professional help.
4' is about the limit for an ordinary wall with with a level area retained with no engineering. With a 45 degree sploe you have much higher loads and can expect movement.
You are fooling yourself if you think you are creating wood "I beams". You just have posts with onle layer or wood retaining the large load and the outer layer doing little but not rotting as fast since it is not in the soil. The way wood shrinks and dries nails will noy create a wall the acts like a big I beam.
Your 4x4s are too small anyway, especially at 36" deep.
you need either tie-backs to dead men or a segmental concrete block retaining wall (not footings) with geo-grid soil reinforcement. Another option is the big Mafia blocks of concrete, but usually there is no guarantee for a site like yours.
A good, long rain could level the 4x4 and 2x8 wall.