Get the last 4 characters of a string

str = "aaaaabbbb"
newstr = str[-4:]

See : http://codepad.org/S3zjnKoD

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.