Hi. DOU literally means "how", (or in your context, "what" as in "what does it mean"). But as in English, in Japanese we don't normally say "that's what it means" but rather, "it means". So it is not necessary to say DOU. So TO is used instead of DOU. Ok, technically, English, one can say "FOUR DAYS is what it means" but in Japanese, we never say something similar like FOUR DAYS DOU IU IMI DESU. In other words, DOU is used only in questions, not in answers or statements. Cheers, - Yan