The Scriptures tell us who Moses is talking about at Deuteronomy 4:35- the Father who raised up the prophet Jesus and sent His servant-son to Israel first (Acts 3:22-26), that is, the one whose voice Israel heard out of the fire Deut 18:15-16; 4:33-36.

Reading:
Exodus 19-20 - out of the fire
Deut 4:35 - He who spoke out of the fire is exclusively the one God and no one else.
Mark 12:28-34 - there is no other but HE is what "the Lord our God the Lord is one" means. Jesus and the Jewish scribe agree how to interpret the Shema by quoting Deut 4:35.
Acts 3:22-26 - God raised up the prophet/His servant-son - necessarily the Father
Deut 18:15-18 - Hence ,the Father is the one who spoke out of the fire who raised up Jesus the prophet
Deut 4:35 - there is no other but He is a reference to the Father who spoke out of the fire
Also read Deut 4:35 through to Deut 6:4 for more insight.

http://www.angelfire.com/space/thegospeltruth/trinity.html