The 4 pins are shorted together in groups of two. They are all used when taking resistance measurements with the 4050.
Two of the pins send the excitation current and two of the pins are used to take a voltage reading which is routed to the voltage measurement circuitry. Because they are shorted in groups of two it is still considered a 2-wire resistance measurement.
When taking Voltage or Current measurements only two of the four pins are used to take the measurement.