The SD Card updateability and the compatibility with almost every Futaba receiver as of late, makes the 4PX worth it. Its interface is better as well, as in evolved to help aid setup and overview. And according to Futabas site the ability to program any S-bus / telemetry via the radio is also pretty cool ( I am not running any telemetry as of yet, but a temp sensor would be awesome). The 4PV is still running the same basic interface since the 4PL.
I am not sure as to speed variation between the two, yet I am sure its performance (4PX) would be noticably better.
https://www.youtube.com/watch?v=qY0p0_ztUnc
Light Comparison:
https://www.youtube.com/watch?v=qY0p0_ztUnc