Why were Yisrael more likely to believe the second sign than the first?
Rashi: Because, when they hear that Moshe was smitten with leprosy for speaking Lashon ha'Ra about them, they will certainly believe him; since Hashem had stricken both Pharaoh and Avimelech with Tzara'as for acting against 'Yisrael.' 1
Seforno: Because to cure Tzara'as that is snow-white (and is akin to death) is a greater miracle than the earlier one.
Gur Aryeh: Why does Rashi interpret this way? Hashem is saying that surely Bnei Yisrael would believe this miracle; whereas He does not state this about the last miracle. There must be something specific about this one - which, Rashi tells us, was that it shows how Moshe was punished due to Yisrael's honor.
Why does it say, "if they will not believe"? Ultimately, it tells us "va'Ya'amen ha'Am" (4:31)!
Moshav Zekenim: That verse refers to the elders that Moshe gathered. The others, however, did not believe.
QUESTIONS ON RASHI
Rashi writes: "They will believe the message of the latter sign' - once you tell them that you were stricken..." While this explains the additional message of this second sign, what remains to be added by the third sign (see 4:9)?
