Can't send text message to a 4 digit number

As soon as i type a 4 digits number (trying to send SMS), the 4 numbers turned to a green color and a "space" between the first 2 number and the 2 numbers is added, finally, i get a message telling "Not Delivered"

i have tried what is posted on the link below, but not working:

https://discussions.apple.com/thread/250174152#:~:text=iMessage%20converts%20a%204%20digit%20number%20to%20phone%20number%20format.&text=imessage%20will%20change%20to%20444%2D8%20which%20then%20will%20be%20rejected

send As SMS is enabled

Thanks

Khaled