Thank you so much. I couldn't have done this without you.
I'm really stupid. Yes, TX and RX are swapped also I should change the baudrate 57600. Then I can get some kind of messages from Pixhawk4.
But still, Raspberry Pi does not find the Pixhawk.
When I type a command as
./takeoff_land serial:///dev/ttyUSB0:57600. Then It said
No system found, exiting.
I tried this on offboard mode, manual mode, stabilize mode.
This is the result of connecting USB to 5pin and running the example.
I think it will work if I fix a little bit.