While Bethesda hasn't officially announced this option, the listing explains that owning the digital PS3 version of Dishonored entitles you to this discounted PS4 version. You'll only be able preorder this if you're logged into an account that owns Dishonored on PS3. At the same time, owning the last-gen version prevents you from purchasing the $40 version, as the $20 option is the exact same game.
It appears this deal is available even to those who own PS3's Dishonored through PlayStation Plus. The game was a freebie back in April.
We've followed up with Bethesda to find out if it will offer a similar discount for Xbox One players who own the Xbox 360 version of Dishonored. We'll report back with any details we receive.