After upload pixhawk firmware(V1.10.1 stable) by Qground(Stable Version 4.0) and matlab(2019b)platform, Qground do not respose to Pixhwak4( CUAV V5). But after uplaod arducopter firmware(V4.0) by mission planner(V1.3.70), the CUAV V5 could connect to mission planner and Qground by usb. The difference between them is MAVlink COM which occured after the arducopter firmware uploaded.
Did you by chance update the bootloader from MissionPlanner?