Building a Retaining Wall Next to Fence
I would not be going 4’ deep. For a small wall like that I would want a wider footing 16” sounds good but would go 18”deep max , I don’t see any way for there to be hydraulic issues that may require a deeper footing. I would have rebar in every other hole and back fill them all with concrete. On the back side I would lay a drain in a sock at the footing or base of the wall and fill around it with rock and cover the rock with landscape fabric. Then the only thing you have is the dirt and 18” should hold up fine possibly 12” depending on how hard it freezes in your area.