As Randy mentioned, while if you have the software downloaded, you will be able to install it, it is certainly not advised/recommended. Not only is this version of our software no longer supported or developed for but much, if not all, of the server counterparts required to run this software are as well. Thus, there are known security exploits, bugs, etc... that will never be patched.
This is really opening yourself up to vulnerabilities and issues. To put this in an analogy, it is very akin to running hotel where criminals know how to bypass the locks on the doors. That is not only opening up liability for yourself but not safe for your guests either.
If you're having any issues with the latest release of version 4, please let us know and we can help with that 🙂 .