I have recently been writing the section of the Galatians EEC commentary on the allegory in 4:21-31. (I actually make the argument that this is not an allegory but an analogy, but that's beside the point of this post.) Figuring out Paul's logic is not impossible, but it is challenging. It feels like a game of whack-a-mole: Once you think you have part of it settled, up pops a new relationship to incorporate. Given my penchant for technical tools, I spent some time in a graphing program mapping out the analogy. It was a helpful exercise, and I'm pleased with the results.
What I wanted to make clear in the diagram are all the different levels of correspondence Paul makes between the biblical narrative and the present situation. His own point is clear: You want to be on the right side of history by being on the right side of the diagram.