strings are immutable (unchangeable). But you can index and join items.

mystring = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
mystring = 'ABCDE'.join([mystring[:20],mystring[24:]])

'XXXXXXXXXXXXXXXXXXXXABCDEXXXXXXXXXXXXXX'

Do be careful as the string length "ABCDE" and the number of items you omit between mystring[:20], mystring[24:] need to be the same length.

For a simpler solution you could instead use rjust on the last 4 characters of the string, and fill it with # up to its original length:

s = 'TestName'
s[-4:].rjust(len(s), '#')

'####Name'

The problem with your function, is that you have to repeat the elements you want to use to replace as many times as replacements there will be. So you should do:

def maskify(cc):
    c2 = cc.replace(cc[:-4], '#'*len(cc[:-4]))
    return c2

If it's always the same position you're replacing, you could do something like:

>>> s = s[0:-2] + "A" + s[-1:]
>>> print s
abcdefghijAl

In the general case, you could do:

>>> rindex = -2 #Second to the last letter
>>> s = s[0:rindex] + "A" + s[rindex+1:]
>>> print s
abcdefghijAl

Edit: The very general case, if you just want to repeat a single letter in the replacement:

>>> s = "abcdefghijkl"
>>> repl_str = "A"
>>> rindex = -4 #Start at 4th character from the end
>>> repl = 3 #Replace 3 characters
>>> s = s[0:rindex] + (repl_str * repl) + s[rindex+repl:]
>>> print s
abcdefghAAAl

A more generic approach would be to split the URL, replace the dot and then join:

In [1]: url = 'www.google.com/bla.bla'

In [2]: s = url.split("/")

In [3]: s[1] = s[1].replace(".", "")

In [4]: "/".join(s)
Out[4]: 'www.google.com/blabla'

As strings are immutable in Python, just create a new string which includes the value at the desired index.

Assuming you have a string s, perhaps s = "mystring"

You can quickly (and obviously) replace a portion at a desired index by placing it between "slices" of the original.

s = s[:index] + newstring + s[index + 1:]

You can find the middle by dividing your string length by 2 len(s)/2

If you're getting mystery inputs, you should take care to handle indices outside the expected range

def replacer(s, newstring, index, nofail=False):
    # raise an error if index is outside of the string
    if not nofail and index not in range(len(s)):
        raise ValueError("index outside given string")

    # if not erroring, but the index is still not in the correct range..
    if index < 0:  # add it to the beginning
        return newstring + s
    if index > len(s):  # add it to the end
        return s + newstring

    # insert the new string between "slices" of the original
    return s[:index] + newstring + s[index + 1:]

This will work as

replacer("mystring", "12", 4)
'myst12ing'