In addition to Tim's answer, which is very appropriate to your specific example, it's worth mentioning collections.defaultdict, which lets you do stuff like this:
>>> d = defaultdict(int)
>>> d[0] += 1
>>> d
{0: 1}
>>> d[4] += 1
>>> d
{0: 1, 4: 1}
For mapping [1, 2, 3, 4] as in your example, it's a fish out of water. But depending on the reason you asked the question, this may end up being a more appropriate technique.