Where can I find old versions of SQL Developer (4.x)?
Oracle removed the links, but the files are still there.
Example: http://download.oracle.com/otn/java/sqldeveloper/sqldeveloper-4.1.3.20.78-no-jre.zip
Of course before using that direct link, you need to go to any other public download page and accept the license agreement, but after that, it works.
Also the full clients contain some old SQL Developer versions.
11.2.0.4 client (for example: p13390677_112040_MSWIN-x86-64_4of7.zip) contains SQL Developer 1.55.
12.1.0.2 client (for example: p21419221_121020_MSWIN-x86-64_7of10.zip) contains SQL Developer 3.2.20.10.
I did not check newer versions.
You can download those files from https://support.oracle.com/, or any random site you can find with a Google search using the above filenames.