First of all, i dont think D3 will get a new expansion, they already scrapped one, i dont think they are willing to work on a new one. I do think they will add new DLC classes but thats about it, the game needs to move forward, its outdated and it needs new ideas for people to invest in.
It might be a very successful game but that doesnt mean people will continue to buy it, all the people who are interested in it have already bought it. It already made its revenue and got its playerbase. Now the only way Blizzard can make money from Diablo is to make a new game so people will buy it again for full price instead of shitty class DLC. And then it can add DLC to D4.
I will buy anything Diablo related, i wont lie, but i will be disappointed if i have to buy D3 expansion instead of a new game. I've already started saving money to buy a new PC for Diablo4 whenever it comes out, so that goes to say i really think its D4 on the works.
You are delusional. There is no D4.
Depending on the news announced on this year's Blizzcon, i might use that as my signature.