PX4 is not a Pixhawk 4. From what I understand, the PX4 platform has been developed along side the APM/Pixhawk platform, although it seems to lag in terms of features and robustness at this point, at least on the software side.
There's a lot going on in terms of 3DR cutting funding to the developers of APM and transitioning to the PX4 platform, which people are suggesting is because PX4 licenses favor proprietary/closed source use as compared with APM/Pixhawk.