The breakdown of the Marvel/Sony deal means that Spider-Man 3 and 4 may not be part of the Marvel Cinematic Universe at all. In early 2015, Marvel Studios and Sony Pictures reached an unprecedented deal that allowed Spider-Man to be rebooted as part of the MCU. Marvel wasted no time in casting Tom Holland as the latest incarnation of the wall-crawler, and he was introduced in 2016's Captain America: Civil War. The new, young Spider-Man has become a hit on the big screen, with Spider-Man: Far From Home becoming Sony's highest-grossing film of all time.
In spite of this box office success - or, perhaps, because of it - the partnership between Marvel and Sony has broken down. Marvel Studios president Kevin Feige was in favor of continuing the relationship, but unfortunately Sony and Disney were unable to reach an agreement. According to Deadline, Disney wanted a 50/50 co-financing arrangement on future movies, and Sony refused point-blank to agree to this. In Sony's view, Spider-Man: Homecoming and Spider-Man: Far From Home have established a creative template that they believe they can follow in order to produce more successful Spider-Man movies.
The key question, however, is just how future Spider-Man films will relate to the Marvel Cinematic Universe. Both Tom Holland and director Jon Watts are reportedly signed up for another two Spider-Man movies, and that gives Sony a couple of options. On the one hand, they could choose to carry out a soft reboot of the Spider-Man franchise, keeping the same stars as well as the same tone and style or move forward with another version entirely, one that's absent of Marvel because of the deal breaking down. Alternatively, they could choose to pick up from Spider-Man: Far From Home's post-credit cliffhanger and continue the wall-crawler's story - absent any future overt references to the MCU.
At this stage, it's unclear what approach Sony will take. Soft reboots aren't exactly uncommon in Hollywood, so the studio could conceivably attempt that. At the same time, though, that's not usually done after a billion-dollar blockbuster hit like Spider-Man: Far From Home. In narrative terms, the cliffhanger ending sets up a Spider-Man film like no other, exploring Peter Parker's life now his secret identity is public knowledge; there's far too much potential in that idea to be wasted. Sony would probably be wiser to take the latter approach.
The question then becomes whether or not Sony would risk contradicting the MCU, letting a developing Spider-Man franchise spin off in its own direction. The studio could choose to avoid explicit contradictions by following the pattern set with Venom, where the script was self-contained and avoided action on a scale that would demand attention from the Avengers, meaning it could quite easily be believed to exist in Marvel's shared universe. The problem with that idea, though, is that sooner or later the MCU will do something on too big a scale for Sony to ignore without it seeming absurd.
Marvel Television has recently faced that same issue, with the cliffhanger ending of Avengers: Infinity War and the five-year time jump in Avengers: Endgame proving to be an insurmountable obstacle for Agents of SHIELD. Sony may well take a tip from SHIELD's experience, and simply allow Jon Watts to do whatever he wants with Spider-Man 3 and Spider-Man 4, regardless of any potential inconsistencies with the MCU.