There is no 3rd movie for 2020, the July date was taken off the calendar. It's just May & November, being Black Widow & either Eternals or Dr Strange 2 (whichever takes November 2020, the other will likely be February 2021).
It takes a minimum of 2 years to make these films. To get an F4 film out for summer 2020 they would have needed to hire a director & writers last summer, and they'd be deep in pre-production right now, casting would have to be well underway, to start filming this summer. You can't keep all of that under wraps, it would be all over the trades.
May or November 2021 is possible, depending on when the deal is completed.
I'm no expert on these big contracts but I'm pretty sure it would be illegal for Marvel to start hiring people to develop a movie they don't have the rights to.
And I know the Russo's had a draft of the CW script with Spidey included before the Sony deal went through, but I don't think that's the same. That wasn't a Spider-Man film, and if the deal didn't go through they could leave him out, no harm and no one would be any the wiser.