Update: Insomniac Games community director James Stevenson has now stated that, in the case of Marvel's Spider-Man: Miles Morales at least, your PS4 save will carry across to the PS5 if you utilise the title's free next-gen upgrade. This situation seems to suggest that cross-save has not been mandated by Sony and so some developers may opt not to support the feature. It also worth noting that the free PS5 upgrade for Maneater also will not carry over your PS4 save — that makes two games then so far. We shall continue to update this article if we learn more.
Original story: Well, this isn't what we wanted to hear. As Yakuza: Like a Dragon confirms that PlayStation 4 save data will not be carried over into the upgraded PlayStation 5 version, Gene Park of The Washington Post has said that this may be the case for many other current-gen titles you'll be able to get a free upgrade for. He says he has heard "a lot from different devs that PS4 saves won't work on PS5. Xbox Series X seems to bring your old save files with you". He then expands: "It's not strictly about [backwards compatibility], it's about PS5 upgrades like Marvel's Spider-Man: Miles Morales [and] Yakuza".
We must clarify that the quote is targeted at PS4 games you will be receiving a free PS5 update for rather than those simply playable through backwards compatibility. This includes the likes of Watch Dogs Legion, Call of Duty: Black Ops Cold War, and Cyberpunk 2077. If this is proven to be true then it means that we would have to start single player games over again, even if we were halfway through a lengthy campaign. There's no better example of this than Assassin's Creed Valhalla. The game releases on PS4 just two days before the US launch of the PS5 — are those who choose to buy the current-gen version and then upgrade maybe a few weeks later going to have to start the game all over again? If someone beats the Cyberpunk 2077 campaign on PS4 and then wants to play the DLC on PS5, what happens there? Surely they would need a fully-levelled character and they won't have access to that.
Again, we must stress that the claim is currently unconfirmed, but after Yakuza: Like a Dragon has announced that will indeed be the case for that particular game, we can't help but worry a little. Let's hope Sony has come up with some sort of feature to convert a PS4 save into a PS5 one because this could quickly become a real problem.