Call of Duty: Modern Warfare 4's release in 2019 has apparently been leaked by the same source who leaked Black Ops 4's battle royale and hero shooter modes. Infinity Ward is set to be the developer and will likely work on singleplayer.
Gaming INTEL is a website that has provided several leaks for Call of Duty: Black Ops 4 over the past two months. These leaks included the Blackout battle royale mode complete with the actual name, hero shooter multiplayer, no singleplayer and the three zombie maps. All these leaks proved to be true later on and were written by the same author, Matthew Harris.
Harris has set his sights on the future of Call of Duty franchise apparently and has recently posted an article claiming that the next instalment in the franchise will be Modern Warfare 4, it will be developed by Infinity Ward and it will be released in 2019.
As reported by Harris, the next Call of Duty title will be "fully fledged", and unlike Black Ops 4, it will have a singleplayer campaign. The regular Call of Duty experience will make a comeback and it will also feature a zombies mode.
Additional "confirmation" of the next Modern Warfare title was also inadvertently hinted by Craig Fairbrass, the voice actor of Gaz from the original Modern Warfare and the superstar of Modern Warfare 2 - Ghost. His bio in March listed four Modern Warfare titles, Modern Warfare 1, 2, 3 and 4 to be specific. He attempted to "clarify" this by saying it was listed due to Modern Warfare Remastered, but all the voice lines in the remaster were actually assets from the original game.
There is also the fact that the developers of the original Modern Warfare recently came back to Infinity Ward in order to work on the next game, which once again hints at the rumours being correct. That said, they are still rumours and should be treated with a degree of suspicion until officially confirmed.
You can find Mathew Harris' article here, and by clicking on the author's name you can see all the previous articles including the Black Ops 4 leaks that turned out to be true.