I would like to prevent input for certain users on a specific UD4. Can I use Data Cell Access Security for this or am I supposed to avoid this?

You could use that, but it quickly gets messy. If the restriction is only input, but they are still allowed to see the members and/or data, I would use conditional noinput rules instead. You can use specific users or groups and say they are not allowed to input to the specific members. Let me know if you need some help with the syntax