(Beraisa - in the name of R. Yudah): If one threw four Amos in Reshus ha'Rabim he is liable.

R. Yudah considers four Amos in Reshus ha'Rabim to be a Melachah by itself. (It is not a Toldah of Hotza'ah from Reshus ha'Yachid to Reshus ha'Rabim. If he did it together with Hotza'ah, he is liable for both.)

Question: According to R. Yehudah, there are 40 Melachos. The Mishnah (7:2, which lists all of the Melachos) should teach this!

Answer: The Mishnah teaches only Melachos that everyone agrees about them.

(R. Ze'ira citing R. Yoshiyah citing R. Yochanan): R. Yehudah learned from people who sewed curtains. They would throw needles to each other.

Question: Was this not [between the curtains, i.e.] in Karmelis?!

Answer (R. Chinena): They threw from the side [of the curtains, i.e. in Reshus ha'Rabim].