The adverb that modified the verb (finished) is quickly.
Adverbs answer the question: HOW IS THE ACTION DONE?
So,
How did John finish his work?
He finished it quickly.
I think it’s 4. Every day in the US, ten people die from swimming.
I most contexts, no. It can be considered an error if viewed from a programmer's perspective, because a bug is viewed as an error in the code of a program. If it is not viewed from a similar perspective, then it is not a synonym. Thesaurus.com can be a helpful place for you.