I’m not sure I understand correctly.
Suppose you are User1, logged on X :0. User2 connects to the machine (this User2 is a different system user). Doesn’t NoMachine display a dialog asking User1 (on :0) whether User2 should be able to connect?
Is there an option for NX4 to open a new session in any case as it was in NX3.5?
NoMachine Workstation and all Terminal Server products do this. They offer the option to create a new session. Anyway this doesn’t solve the problem of what to do when User2 wants to connect to the desktop of User1. This is a perfectly common case of desktop collaboration.
At the moment the solution is to let User1 connect without asking permissions (since we assume User1 is the same User1 and asking permissions would block User1 from connecting in the case he moved to a different machine), and force User1 (or any other user currently viewing :0) to authorize User2.