Even simpler than dkl65's example is to use 4 different single letters
I predict that PaulS will say "Then use G,H,I,J!". 
I will have time on Saturday to test codes on my Arduino UNO SMD edition. That is why I am so unsure of my examples!
If you have time.Could you create at least part of my code
I think that I have already done that. 
Look at one of my other posts through the last link on my first post here (Serial.read crazy results - #20 by system - Programming Questions - Arduino Forum). It's not only you that's having problems reading multiple bytes of Serial data! Serial data arrives asynchronously (one byte at a time [correct me if I'm wrong]). Therefore, we have to use a while loop to loop until all the bytes of Serial data is read. In the while loop, inserted each byte into a string as a character. Outside the while loop, check if the string is empty. If it isn't, use further conditions to check is "X0", etc. and do something if it is. After all the conditionals, clear the string for new input. Looking at the link, you should know how to modify it to do the aforementioned (most steps are already done for you there).
P.S. I'm a Full Member! 