@GKSeifert: The PW4 (the new water resistant one) is called kindle_all_new_paperwhite_v2 in the update files. Just like the PW2 is designated as kindle_paperwhite_v2 as opposed to kindle_paperwhite for PW1 and kindle_all_new_paperwhite for PW3.
Naming convention is what it is.
And ilovejed beat me to it, see post above.