After his electricity went out, Mr. Choi took out two candles. Both were the same length, but the label on candle A said it would burn for 4 hours while the label on candle B said it would burn for 5 hours. Mr. Choi lit both candles at the same time and let them burn until the electricity was restored. He discovered that what remained of one candle was exactly four times the length of what was left of the other. How long were the candles burning?