My father has a swollen 4 to 5 cm mass in his chest, it is a soft mass, what can be the cause of this? He also has swollen lymph nodes in his neck.

Needs a medical chec. Any lump or growth in the body requires a medical check up in order to make sure that it is not a cancer. So I would advise you to get your father to go see his PCP for a medical check up and possibly some tests to define the nature of this mass.