@DoEon_Lee: I don't think the problem lies with the PX4 F/W itself, I think it has to do with your configuration of NuttX for the STM32F446 chip. Unfortunately there a number of things that could be different between your F446 and the standard F427. I know there is a difference between the USB registers for the F446 and the F427, so I really hope that the correct USB registers are being used in your compilation.
Also, if you are using a "standard" defconfig for the px4fmu-v4, then you do not have the console on the USB, so it will not show up on your computer.
The following lines will need to be set
This should put the NSH console on the USB port. This might still not work, owing to difference in USB register between the F446 and F427, but it is worth a try.
When you recompile, make sure you run
make clean first, to make sure that your new config files are used for the compilation.