windows subsystem for linux – Serial port to COM port – no output found

In WSL 1, COM4 should be automatically mapped to /dev/ttyS4 as far as I know. Since WSL 1 is just an API wrapper on top of the Windows kernel (like an inverse of Wine) there might not be any messages about it in dmesg.

So, try ls -l /dev/ttyS*, or setserial -g /dev/ttyS*. Note that the device name is case sensitive, so the S must be upper case.

On the other hand, WSL 2 apparently won’t support regular serial ports at all, but up-to-date versions of it can support USB-serial converters.