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.
Blizzard post: