As was already mentioned, it doesn't really matter what core count computers have, it matters what Dev's want to do.
It also matters what their game engine supports. They wont develop new engine for just increased core count.
For big companies, consoles are big enough market that their core count dictates game's core support.
As was noted, new line-up will have more than 4 (8 to be exact) so they might jump from 4 to 8.
Then again, old xbox one had also 2x4=8 but half of them were reserved for system operations and only 4 for game usage.
Is same repeated on new xbox one x? maybe, maybe not... no specs given yet.
I would assume that 1 to 2 of said cores will be reserved for system usage to make the underlying things work well and rest (6?) could well be usable for games.
New games/engines need to be developed though so as UsaFret said, likely in 2020 or so? earlier ones will likely be in 2018 or 19 but it wont be commonplace.