。
She accidentally skipped the fourth question on the exam.
Literal
She [topic-は] exam [at-で] fourth-question [object-を] ended-up-skipping.
The auxiliary ~てしまう (casual ~ちゃう) marks completion with an emotive nuance — typically regret, accident, or unintendedness. Here it captures that the skipping wasn't deliberate; she missed the question by mistake. The compound 四問目 stacks 四 ('four') + the counter 問 (for questions) + the ordinal suffix 目 — '(the) fourth question.' The same 目 makes 一番目 'the first,' 二回目 'the second time,' 三日目 'the third day.' で marks the exam as the setting where the skipping happened.