Decks, Patios, Porches and Docks - porch support w/ retaining wall

I need to replace an insufficient support on my front porch. The front right corner is the only corner that is not attached to the foundation of the house, and is supported by a hollow metal column, which has a 6" deep rock below it. That rock rests in dirt held up by a 2' high retaining wall.

The porch has begun to sag a bit in that corner, so I'm planning on jacking it up, putting in a quick-crete building form filled with concrete with a 4 x 4 support.

My question is this: how far down past the bottom of the retaining wall do I have to dig for the concrete support? I could only find 4' long forms, so I'm assuming that is the idea distance. But, is that 4' from the top of the retaining wall, or from the bottom?

If its from the bottom, I'll have to use 2 forms directly on top of one, another, is that correct?

I'd really appreciate some guidance here. Thanks!!

The depth required depends on the frost depth in your area. Call the AHJ and ask for the frost depth.