possessives - What's the genitive of "someone else"?

This is Konrad.
He has a dog.
Hence, it's Konrad's dog.

This is someone else.
He has a cat.

1. Hence it's someone else's cat.


2. Hence it's someone's else cat.


3. Hence it's someones else cat.


4. Hence it's someone elses cat.

I know that I can say "it's a cat of someone else" or "it's a cat belonging to someone else" but that dodges the issue, not answering the question.

Answer

According to The Cambridge Guide of English Usage, so well established are phrases like "someone else", "anyone else", "what else" and "who else" that else can take the possessive form quite easily.

In the light of the above reference the possessive of "someone else" is "someone else's".

It is worth noting that "This usage was once frowned on by those who insisted that else was an adverb and so could be made possessive", and this presumably justifies your doubts.