5.0.2 is the latest version of the Mac driver. All of the latest Mac software is under the OSX heading on the driver page.
I downloaded both Mac OS_X_HP Designjet 'Driver.dmg' and 'Printer Utility.dmg' yesterday. I ran both programs but didn't bother to check the driver version until now. It is still 4.2.1, not 5.0.2.
Then I re-ran 'Designjet Installer' (one of two programs when you unpack 'driver.dmg). And, this time I read 'Read Me' carefully, and it states that it is Postscript Drivers.
So, I guess they don't have the latest driver for non-Postscript printer.
In conclusion, I am running with v126.96.36.199 firmware with v4.2.1 printer driver.