Why does the Pasuk use the word "Shafal" and not "Amok" (See Rashi) as it did in Pasuk 3?
Ramban (in Pasuk 3): This is because the Nega is not pure white, but white mixed with red, which therefore does not appear as deep as a Baheres on plain skin, which is pure white.
What is the significance of the lengthy statement "ve'Tim'o ha'Kohen, Nega Tzara'as hi!"?
Ramban (on Pasuk 3): The Kohen should immediately declare it Tamei, since it is a plague that will definitely turn into Tzara'as.