That and the ability to use an external power adapter, if desired.
Also, one can play with the configuration of the ground channel with different opamp combinations in the P3+.
For those who like to experiment with various architecture configurations and opamp choices, the P3+ is a good choice for a basic amp.
While the D4 lacks the ability to use an external power adapter and has a fixed ground architecture, the dual DACs and excellent amplifier quality make it a good all-around choice.
FWIW, if I was forced to keep only one of my iBasso amps, it would be the D4.