To determine the mass number of a nitrogen atom with 4 neutrons, we need to consider the number of protons and neutrons in its nucleus. Nitrogen atoms typically have 7 protons. So, the mass number can be calculated as follows:

Mass Number = Number of Protons + Number of Neutrons

Mass Number = 7 (protons) + 4 (neutrons)

= 11

Therefore, the mass number for a nitrogen atom that contains 4 neutrons is 11.