Here is a brief overview of the timeline for this patch (full details linked below):
14 JAN 2020
Titan Residium will convert to silver today with the release of the patch.
Titan Residuum will not drop from any source for one week. This includes scrapping or disenchanting Seasonal gear.
Mythic Keystone dungeons are still playable using Season 3 affixes, and their associated rewards remain available through this week.
21 JAN 2020
Season 4 begins.
Dungeon difficulty is increased for Heroic and Mythic difficulties.
Mythic Keystone and PvP chests that are opened this week reflect Season 3 rewards earned the prior week, and cannot be Corrupted, but may Warforge/Titanforge.
Titan Residuum is again available.
28 JAN 2020
Weekly chests for Mythic Keystone and PvP activities now contain Season 4 items, which are always Corrupted, and scale up to item level 475. Again, these items will never Warforge/Titanforge.
I’m afraid we really wouldn’t have anything that isn’t otherwise listed in the patch notes, Partygirll.
The current note in there for the Titan Residuum.
All Titan Residuum will be converted to silver.
This is from the Visions of N’Zoth Now live! Blog.
Battle for Azeroth Season 4
Battle for Azeroth Season 4 begins, January 21 and includes an all-new Affix—Awakened— for Mythic Keystone Dungeons, Heroic Difficulty Darkshore Warfront, and more.
I’d just check out the available blogs for info. I have a collection of those threads in the sticky at the top of the page.