I have finally had a chance to figure it out… the silk screen on the 4 pin header is not correct as per the image
go take a loook here Serial Port device names at what I and @tcbetka figured 0out the past few days.
it’s been acknowledged by seeed that there is an error and should be corrected in the wiki already, but i proved the correct device names.
Basically
SerialUSB is the serial monitor (still has issues sometimes locking up the ide)
“Serial” is the 4 pin header
“Serial1” is the 28 pin header D0/D1
/dev/ttyS4 is the 40 pin header device accessible from linux.
/dev/ttyACM0 is the linux device node that is :“SerialUSB” so Theoretically, you could write SAMD code to use the Serial SUB library and talk to it via /dev/ttyACM0 and still have 3 serial ports available 