Like this:
>>>mystr = "abcdefghijkl"
>>>mystr[-4:]
'ijkl'
This slices the string's last 4 characters. The -4 starts the range from the string's end. A modified expression with [:-4] removes the same 4 characters from the end of the string:
>>>mystr[:-4]
'abcdefgh'
For more information on slicing see this Stack Overflow answer.