In your loop, the first port.read() is dumping a character, it's not saved, and therefore lost.
Also, I would make the moteino loop delay shorter than the delay you have between writes in the Pi code. In general, you want your receiver to gobble faster than your transmitter.
I think your original code in the Pi was ok other than the fact that you were not delaying between writes, so:
import serial
from time import sleep
ser = serial.Serial ("/dev/ttyS0", 19200) #Open port with baud rate
while True:
ser.write(b'\x43\x44\x45\x46') #transmit data serially
sleep(2.5) # time for 4 byte read plus 'extra'
Also, given that the Moteino loop checks to see if anything is available, the delay in that loop is unnecessary. Reads (and debug outputs) will be synchronized to writes.