You cannot activate v3 with a v4 license unfortunately.
What exactly is not working for you with v4? You will need both ends to run v4 as a v4 will not connect to v3. Typically the best thing to do is uninstall from both ends, REBOOT (important) and then install the new version followed by a reboot ideally again.
Worst case I believe v3 is still available from some third party suppliers for example or support may be able to assist changing things for you, but they will not be in until 9am EST.
One place that seems to have it :
https://www.fanatical.com/en/game/multiplicity
That is MP3 KM if that was the version you were using rather than KVM (the more expensive version with KVM as well as Seamless)