Opposite is a very flexible word. It can be used as an adverb and as a preposition. I’d say both 1 and 4 are correct, but 1 is more idiomatic: in situations such as this, the shorter form is usually preferred.
I agree. ‘Opposite of’ wasn’t on offer in the question. Maybe a better example of its use would have been: “this was completely opposite to how she had imagined it”.