The Western Electric 4D is a six tube (215As) superhet. It is designed to work with a loop antenna or a long wire. The 2A is the antenna tuning unit that could be paired with any of the 4 series receivers. Technically, though, the manuals/brochures of the period only pair the 2A tuner with the 4C receiver, and the 2B tuner with the 4D receiver.