Midrasch zu Schemot 38:2

And he made the horns thereof upon the four corners of it (Exod. 38:2), in order to obtain forgiveness for the nation that exalted Him through four horns. The horn that came from Sinai, as it is said: And He hath lifted up a horn for His people (Ps. 148:14); the horn of the Torah: Horns hath He set at His side; and there is the hiding of His power (Hab. 3:4); the horn of the priesthood, as it is said: And My horn hast thou exalted (Ps. 92:11); and the horn of kingship, as is said: Moses knew not that the skin of his face sent forth beams (horns) (Exod. 34:29). And it is written elsewhere: And there was a king in Jeshurun (Deut. 33:5).