I am at vol 3 ch 20. Got mail today talking about opening ch 4, what and where is this?

It is announcing the Flash Bonus in the Help Wanted Challenges. You'll randomly get this limited-time bonus if you have reached Chapter 4 of the first volume (~level 16). For a short time period, you'll need to complete HW cases to get the bonus reward that is displayed. You'll recognize it by the red circle around your HW icon. The length of the circle arc also indicates how much time left before the bonus is over.