It sounds like the build on your computer you switched to be a primary is an earlier beta build that had an issue with limiting the number of active secondary computers to one at a time when using a Multiplicity 4 non pro licence.
This was resolved in the release version so I would recommend uninstalling it on that computer, rebooting and then downloading the latest version on your account and installing that.
We also have a pending update for Multiplicity 4 which I hope will be going out in beta next week with some tweaks to various things based on customer feedback. But your specific issue should be addressed already in the release version so you just need that.