Do I have to apply 4 lines at the same time to get discount?

Take AT&T UNLIMITED ELITE for example, we can get discount if we apply more than 1 lines (2, 3 or 4 lines and above). I wonder if I should apply 4 lines at the same time to get discount? If I apply 2 lines at the beginning, and then invite another two family members to join AT&T plan, can I still get 4 lines discount?

Another question is, is the discount apply to both online and physical store?