Charles Ellicott Commentary Exodus 4:16

"And he shall be thy spokesman unto the people; and it shall come to pass, that he shall be to thee a mouth, and thou shalt be to him as God." — Exodus 4:16 (ASV)

He shall be thy spokesman. —Hebrew: He shall speak for thee.

He shall be, even he shall be. —Rather, it shall come to pass that he shall be, etc.

Instead of God. —God did not speak to Aaron directly, but only through Moses. Aaron was to recognise in Moses God’s mouthpiece, and to consider what Moses told him as coming from God. Moses still had, therefore, the higher position.