I only have the option of buying on ebay, but depending on your location, you might at least be able to return the PW2 if it comes with FW 5.6.x.
Anyhow, I've been looking at ebay, and I seem to have found out that the PW2 4GB can come in two different looking boxes which is interesting because it might say something about the possible firmware versions on the device.
http://i.ebayimg.com/00/s/ODc5WDY0Mw...UJw-5/$_57.JPG
http://i.ebayimg.com/00/s/ODM1WDcyMA...UiYGX/$_57.JPG
Edit:
I found this firmware/serial comparison from a German article comparing the 2013 2GB version to the 2014 4GB version. Both uses FW 5.4.5 in this case. So it must be possible to acquire the PW2 4GB with an jailbreak-able firmware.
http://allesebook.de/wp-content/uplo...icherplatz.jpg