Only sentences #1 and #3 are grammatically correct although #1 is the most correct.

Sentence #3 is somewhat gauche but it means that “someone” whose identity is inferred has also eaten your porridge on top of something else. It’s very stylistic and seems to confer a sarcastic meaning.

Sentence #2 is grammatically incorrect. Aussi is an adverb and that’s simply not where you put them in French.

Sentence #4 is somewhat grammatically but does not make sense, because it is not syntagmatically correct.

The four examples are correct, I will add some names to clarify the meanings.

1) This one has the same meaning than in English:

Paul says that someone ate his porridge, then Anne says “Quelqu’un a
aussi mangé mon porridge !”
meaning that her porridge has also been eaten by someone.

2) This “aussi” would be translated as “too”. It is correct grammatically but like the English “Someone too had eaten my porridge!”, it doesn’t really make sense (it’s possible but farfetched). If you replace “Someone” with a name it becomes clear:

Paul says that Jean ate Anne’s porridge, but Anne says “Louis aussi a
mangé mon porridge !”
meaning that both Jean and Louis enjoyed her porridge.

3) This time it can mean both “also” or “too”, with either the same meaning as 1) or:

Anne says that someone ate her apple, but notices later that someone
ate not only the apple but her porridge too. “Quelqu’un a mangé mon
porridge aussi !”

4) This is another meaning of “aussi”. When placed at the begining of a sentence it introduces a causality link with the previous sentence, and can be translated as “therefore”, “hence” or “as a consequence”:

Anne says that there are three ever-hungry teenagers in the house, and
she forgot her porridge on the table. “Aussi, quelqu’un a mangé mon
porridge!”

However his meaning is quite formal and you will probably not hear it in casual speak.