Why PyPDF4? Why not just stick with PyPDF2?

[... eventually explain more ...]

Short version: PyPDF4 is a clean break designed to do what PyPDF2 did, but on a more sustainable, business-worthy basis. Yes, in principle we could have just reconfigured PyPDF2 until it arrived where we want PyPDF4 to be. Our judgment was that the cost of leaving PyPDF2's assets and liabilities behind was less than the gain from a fresh organizational start.