pypdf4 is a fork of pypdf. pypdf4 had its only release in 2018, whereas pypdf is maintained again.
PyPDF4 is a fork of PyPDF2 which is a fork of pypdf.
PyPDF4 is dead. Please only use this tag if you are using PyPDF4. The pypdf project has its own tag.
In December 2022, PyPDF2 was deprecated in favor of pypdf