I've thought about calling them to ask why this one didn't have 4Gb but:
-they don't advertise the PW as having 4 Gb (the web site still says that it stores over 1000 books); and
-my old one had 2Gb and I never came close to maxing the storage (having collections sync across devices means I only store a few books on the device now).
I got this one with a 10% discount for having an AARP membership, so I've wondered if they were using the old stock for those orders.
If someone does care whether it has 4 Gb, I'd suggest calling before purchase to make sure it's going to have it, or maybe wait until next month to see if they announce a new version. (I'm replacing one I lost and didn't want to wait that long. The only thing that would make me buy a new version would be physical page turn buttons.)