The main statement says an operating system failure guarantees the computer will not work, in short, OS fails implies computer does not work.
The only safe reversal is the contrapositive: if the computer is working, the operating system cannot have failed, giving 'computer works' leading to 'OS does not fail', exactly statements 4 and 2 in that order.
The remaining pairs either contradict themselves directly, reverse the original conditional's direction, or claim something the main statement never actually guarantees, namely what happens when the system does not fail.
Therefore, the correct answer is 4, 2.
"The computer will not work if operating system fails" tells us that an operating system failure is enough on its own to guarantee the computer stops working, it is a sufficient condition for the computer failing.
Since an operating system failure is sufficient to stop the computer working, a working computer rules out that failure entirely, exactly the pairing of statements 4 and 2.
Therefore, the correct answer is 4, 2.