D4 is quite literally years away. At least three is my guess, closer to 4 most likely. What we saw is pre alpha. 2020's blizzcon most likely will show us at least 1 of the remaining classes to be announced, probably both. 2021 Blizzcon will either show the 2nd, or just the rest of the systems they decided on. Might get some beta news after, and if we're lucky a launch sometime late in 2022. 4
Yeah I would bet money on both classes being playable next year at the con. I can't imagine them not having a playable class and having to develop itemization, combat, etc., with only three classes. I feel like it would be like the baseline for moving forward on many fronts.
Also agree with @WishesHeHadBeta on the timeline. Makes sense on many fronts.
It's also usually the case that what we see at BlizzCon is usually a bit behind what they actually have internally. For instance, the next wow raid area they showed off in one of the panels was already datamined showing a lot more progress before the event.