You'ld probably have to go quite deep for the sonotubes and put a lot of rebar in them. The soil has a tremendous amount of pressure and can easily tip over retaining walls that aren't designed right. The soil pressures will depend greatly on the type of soil you have. Almost everyplace requires engineering for retaining walls 4' high.
But, have you considered a modular block retaining wall? They would be easy to move under your deck. They don't require a concrete base and can be layed with minimal excavating (usually placed on some compacted gravel). I'm pretty sure the manufacturers have engineering data to support using their product at appropriate heights.
Another thing to consider is breaking that 4' rise into two 2' rises spaced 3 or 4 feet apart. I would think modular blocks would easily handle 2' of nearly any soil conditions.
But, have you considered a modular block retaining wall? They would be easy to move under your deck. They don't require a concrete base and can be layed with minimal excavating (usually placed on some compacted gravel). I'm pretty sure the manufacturers have engineering data to support using their product at appropriate heights.
Another thing to consider is breaking that 4' rise into two 2' rises spaced 3 or 4 feet apart. I would think modular blocks would easily handle 2' of nearly any soil conditions.