Update [11:57 p.m. ET]: Cuphead has, in fact, launched today on the PS4. At Geoff Keighley's Summer Games Fest, the full stream of which you can check out just below, the immediate launch of Cuphead on the PS4 was revealed.
It certainly looks like Cuphead will be making its way to the PlayStation 4. A leak through the PlayStation Store seems to point to the former Xbox-exclusive game making the jump to the new platform.
Overnight, a ReserEra user noted that Cuphead had suddenly appeared in the 'New' games section of the PlayStation Store in the UK, with additional users also reporting the game appearing on their stores in various other regions. Checking out the UK store for myself this morning though, it appears the game listing has been wiped from the store entirely.
As VideoGamesChronicle notes, this could be what Geoff Keighley has lined up for the Summer Games Fest showcase later today at 8 a.m. PT/11 a.m. ET. Keighley has said he'll premiere a "special update from one of our favorite indie game studio," and developer MDHR certainly fits that bill.
Additionally, you might have noticed there was a listing for Cuphead's The Delicious Last Course DLC recently on Xbox.com. The listing in question said "available now," speculating the DLC would be released alongside the PS4 port, but this now appears to have been an error on the part of the Microsoft Store.
Cuphead has proved a certifiable indie hit for studio MDHR since it launched in 2017. Releasing as an Xbox console exclusive while also arriving on PC, Cuphead has shipped a staggering five million copies worldwide since launch, eventually arriving on Nintendo Switch, and now PS4, it would seem. It's so popular that there's even an animated series adaptation in the works over at Netflix.