While I'm waiting for my replacement Mavic, which's coming next week, I am wondering if any of you have tried to connect multiple mobile devices to the remote controller using a USB hub, as a way to share the screen and control the drone using multiple devices running DJI Go 4.

Unfortunately no. When the power port is connected to a device, the lower USB port is disabled. So you can not run two GO4 apps with a single controller.

However, beginning with Mavic firmware 01.03.0400, along with DJI GO4 app, iOS version 4.0.5 or Android version 4.0.4, the Mavic Pro will support DUAL REMOTE CONTROLLER operations. If you have or purchase a second controller, they can be operated as Master and Slave. See the Mavic Pro User Guide v 1.4 page 37 for further details....

Good to know. Thanks.