I think the answer is 「４．何かで」
I guess it goes like:
たまご りょうり の じょうずな つくりかた を（何かで）読みました。
「で」 is "by"
So, it can be by a book or a magazine.
The translation of the entire sentence'd be like,
"I (did) read something how to cook eggs well."
And it implies he/she doesn't remember what it was lol