This is the primary legitimate way to get older versions. If you have an active Visual Studio Subscription , you can log in to the subscriber portal to download the ISO and product keys for legacy editions like 2013.
While originally a free, full-featured version for non-enterprise developers, it now generally requires a free Dev Essentials subscription to access the download files. The Story of its Launch (2013) buy visual studio 2013
Description of Visual Studio 2013 Update 4 - Microsoft Support This is the primary legitimate way to get older versions