4a - In one case the Gemarrah discusses an itztabah which is less than 4 amos away from an opposite wall and it teaches that dofen akumah works with that wall opposite the itztabah even though it is above 20 amos. Later, the Gemarra discusses another case of an itztabah which is less than 4 amos away from walls in each direction and it teaches that dofen akumah works with those walls opposite the itztabah even though they are above 20 amos. Why doesn't the Gemarra just bring the last case with 4 walls and we would certainly know the earlier case with just one?
Avraham Sacks, Ramat Beit Shemesh
1) We can say that this is an example of "Lo Zu Af Zu" which we often find in the Mishnah. This means that first the Tana says a smaller Chidush and then says a bigger Chidush.
2) Rabeinu Shimshon (one of the Rishonim) writes in Sefer Kerisus (which deals a lot with the rules of the Gemara) in part 5, Lashon Limudim Sha'ar 2, chapter 90 (concerning "Lo Zu Af Zu") that the Tana of the Mishnah wrote down the Halachos in the order that he heard them. If he heard the smaller Chidush first, then he wrote it down, and when he heard the second Chidush, he wrote that down too. Even though he later could have made it suffice by saying only the second Din and leaving us to work out the first one from the second, the Gemara did not want to do this because of the rule "Mishnah Lo Zazah mi'Mekomah" (see Chulin 32b for example); we do not change the original text of the Mishnah.
3) The aforementioned source is chiefly referring to the Mishnah. However, we can extend this idea to the entire Gemara, so our Gemara will be in the style of "Lo Zu Af Zu."
Kol Tuv,
Dovid Bloom