Commentary for Pesachim 187:15

Rabbi Yehuda says: From sunset to dusk one can walk four mil - Rabbeinu Tam finds this difficult, since at the end of [the chapter named] "With what do we light" (Shabbat 34b) it says: "What is twilight? From sunset for the whole time that the eastern sky is red - these are the words of Rabbi Yehuda." They conclude there that Rabbi Yehuda's twilight is three quarters of a mil, and here Rabbi Yehuda himself says it is four mil. One can [solve this by] saying that here he was stating from the beginning of sunset, which is when the sun begins to enter the width of the firmament, until the night time, which is four mil. There he was counting from the end of sunset.