The Arknights Twitter account announcing the arrival of Chapter 5: Necessary Solutions!
Current speculation is that they will launch it from sometime between 24-27th, as the Japanese Arknights site shows a special animation with an increasing number of slashes each day. As it currently has 4 slashes, the theory is that Chapter 5 will drop once it reaches 7 or 10 slashes (the number of strikes Ch'en's S2/S3 does respectively), or 3-6 days from this post.
CN Launch Campaign Part 1
On the CN Server, the launch campaign had two parts. Some of the banners and campaigns were as follows.
3 New skins will be available for sale - expect a "Should You Purchase" soon!
Two new furniture sets are added: one permanent, and one limited.
New challenge mode quests will be available after clearing Chapter 5!
All Supplies and Chip Missions will be available every day, for a week starting from Chapter 5's Launch campaign!
CN Launch Campaign Part 2
Similar to the current one with Perfumer's skin, this sign-in campaign one offers login bonuses and a Myrrh Skin!
These Challenge Quests will have required beating the previous ones to unlock them.
Again, the All Dailies Open will run for a week after Part 2 begins.
Clearing Lungmen 3 will also increase the weekly Orundum cap.