The best way to obtain an older version is to contact us via the website. Backward compatibility is generally maintained between versions.
Customers who have subscriptions with us can put in a special request to receive a download link to the earlier version if they are not able to update the server, or if they are still using NoMachine login even with version 5 (or 4). They can then distribute it to their users privately. Perhaps your administrator has already been in touch with us? Either way, please reach out to us using this link: https://www.nomachine.com/contact-request.