If I purchase 3 Pro accounts and 1 Collaborator account is this set up as a Floating License model — meaning the accounts are all linked under my email, and I can have 5 users in my team, but only 3 of them can actively use/modify at the same time? Or is it strictly a Named User license where each Pro seat must be permanently assigned to an individual email?
Yes, you can have 5 users and re-assign any of the Pro or Collaborator licenses to specific users under the "Users" page. You just change who has the licenses using a dropdown. Workspace owners or administrators can re-assign the licenses.

Thanks for your reply, just to clarify, say I allocate all 5 user to the 3 Pro Licenses, it will be 1st 3 to login will have access (similar to Planswift), or would the admin need to login and change licenses as required each time
Currently an admin would need to re-assign the licenses, but it's pretty quick with the dropdown. I can see that would be beneficial to have a setting for automatic license removal, assignment when users open/close the software as long as it limits to the total # of licenses purchased.