Past continuous is often used to contrast a long action (eg. wait) to a short event (call). This is why 1. is correct.
Past continuous is also used to describe a long action (eg. clean) at a specific point in time (eg. 4:00, yesterday). However is usually does NOT compare a long action to a duration (eg. for four hours, in 5 minutes). Therefore past simple works better in your sentence.
Another way to look at it is that the question contains a past simple verb, therefore it sounds strange to switch to a different tense in the answer.
If you said "She was cleaning for four hours" in spoken English, I doubt anyone would pay much attention to the "mistake", as it's very minor.