Squawk codes are different for regular flights, ATF use them to identify you on their screens. For regular flights they are different every flight. When communicating with ATF tail numbers are used for communicating (abbreviated normally) my plane tail became 71 Yankee when ATF gave instructions. After taking off and contacting flight center they sometimes would change squawk number.
Squawk numbers for us regular guys when given by ATC or flight center were always 4 numbers, no letters
Ops, ATF should be ATC.