Having promised Yisrael bread in Pasuk 4, why does Hashem see fit to repeat it here?
Ramban: In Pasuk 4, it seems that Hashem was promising them bread purely out of loving-kindness on Is own volition or out of Yisrael's merit. Therefore He points out here that He was doing so in answer to their complaints, issuing a rebuke for their lack of faith in Him and for grumbling to His prophets.
Why does Hashem repeat here what Moshe already said in Pasuk 8?
Ramban: From Moshe's initial statement, the people probably understood that the Manna would fall once or twice, and that once they moved on, they would arrive at a location where bread was available. Hence Hashem now informed them that, for the duration of their stay in the desert, the quails would fall every night, 1 and the Manna, every day.
Indeed, Chazal maintain that the quails, like the Manna, fell throughout the forty years that Yisrael were in the desert, and the reason that the Torah deals at length with the Manna exclusively is due to its miraculous nature (Ramban [Refer also to 16:6:1:2]). See Tos. Erchin, 15b DH 'His'u' - Rav Chavel's commentary on the Ramban. The Ramban also discusses the Parshah of the quails in Bamidbar (See 11:4), in view of the fact that the quails were already falling each night. See also Ba'al ha'Turim on Pasuk 13.