Just in case you ever need to use the field calculation in a Python script, to do the same thing in Python you would also use the len() function. For example:
!input![len(!input!)-4]
or
rstrip(!input![len(!input!)-3])
(Keep in mind that whitespace characters count, so in your examples "BN14 5RY" is 8 characters long, not 7 and "BN1 5RY" is 7 characters long, not 6.)