I'm not getting a clear picture of what the problem is. By "leaning" do you mean sagging/dropping or do you really mean leaning as in the porch is racking/leaning to one side?
If the lower wall is 4 feet deep, it should be stable & not need a (new?/additional?) footer.. but again, I'm not sure if I'm visualizing it correctly.
Can you just support the porch with jacks and rebuild what has failed?