Who were the four men?
Rashi (from Sanhedrin 107b): They were Gechazi and his three sons. Malbim - Elisha healed the other Metzora'im. He healed Na'aman (a Nochri), and all the more so [Yisrael] Metzora'im 1 ! Also the girl who told Na'aman about Elisha, surely she knew that he normally cured people!
What is his Kal v'Chomer? A Yisrael should understand that Tzara'as is a punishment; when he repents, he will be cured! (PF)
Why were they at the opening of the gate?
Rashi: "Badad Yeshev mi'Chutz la'Machaneh Moshavo" 1 (Vayikra 13:46). Radak - they would normally be [just] outside the city.
Malbim (citing Kelim 1:7): This forbids them to be in a walled city. (Really, "Badad Yeshev" also forbids Metzora'im to dwell together (Zevachim 117a)! - PF)
What is the meaning of "Ad Masnu"?
Radak: It is like Ad Asher Masnu (until we will die).
