Each PC and each Reader will count as a single device (of your allowed 6), so you're right on target there -- you can create a single account and put all 4 devices on it and still have 2 spots free.
If you take a device off (de-register it from the account) then its slot opens up. You have to have the device physical handy to do the de-registration, so if you know you're getting rid of one of the PC's, for instance, de-register it
before you get rid of it.
Don't worry if you can't do that, though, because you can always call the customer support folks and ask them to de-register a device -- we had to do that when the home PC crashed in January.
They were able to take it off for us though, no fuss, no muss.
As far as I know no one's cracked the DRM yet, but the hopeful news in that area is that Sony has signed on to go with Adobe's Digital Editions. If that takes off, then we may get a de-facto portable format for e-books out of the deal.
