Specially. Figure of speech Ellipsis. App-6. Might be supplied by the word "Remember".
Horeb. Occurs three times before Deut. (viz. Exodus 3:1; Exodus 17:6; Exodus 33:6). Sinai is the general word, but occurs only once in Deut. (Deuteronomy 33:2).
Related Commentaries of Deuteronomy 4:10
Deuteronomy 4:10
10 Specially the day that thou stoodest before the LORD thy God in Horeb, when the LORD said unto me, Gather me the people together, and I will make them hear my words, that they may learn to fear me all the days that they shall live upon the earth, and that they may teach their children.