Your problem is with this line:
number4 = list(cow[n])
It tries to take cow[n], which returns an integer, and make it a list. This doesn't work, as demonstrated below:
>>> a = 1
>>> list(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>>>
Perhaps you meant to put cow[n] inside a list:
number4 = [cow[n]]
See a demonstration below:
>>> a = 1
>>> [a]
[1]
>>>
Also, I wanted to address two things:
: at the end.input like that, since it evaluates its input as real Python code. It would be better here to use raw_input and then convert the input to an integer with int.To split up the digits and then add them like you want, I would first make the number a string. Then, since strings are iterable, you can use sum:
>>> a = 137
>>> a = str(a)
>>> # This way is more common and preferred
>>> sum(int(x) for x in a)
11
>>> # But this also works
>>> sum(map(int, a))
11
>>>