Lets take a deeper look at what is said through Malachi
Remember the Law of Moses, My servant, Which I commanded him in Horeb for all Israel, With the statutes and judgments. (Malachi 4:4 NKJV)
So he asks us to focus our attention to the time of the commandments Which is Exodus 20. When they said.
Then they said to Moses, “You speak with us, and we will hear; but let not God speak with us, lest we die.” (Exodus 20:19 NKJV)
Deuteronomy then speaks again of this event
“The Lord your God will raise up for you a Prophet like me from your midst, from your brethren. Him you shall hear, 16 according to all you desired of the Lord your God in Horeb in the day of the assembly, saying, ‘Let me not hear again the voice of the Lord my God, nor let me see this great fire anymore, lest I die.’ 17 “And the Lord said to me: ‘What they have spoken is good. 18 I will raise up for them a Prophet like you from among their brethren, and will put My words in His mouth, and He shall speak to them all that I command Him. (Deuteronomy 18:15-18 NKJV)
So let us look deeper at this next line in Malachi
הנה אנכי שלח לכם את אליה הנביא לפני בוא יום יהוה הגדול והנורא (Malachi 4:5 Hebrew OT: WLC (Consonants Only))
SEE (הנה)[Particle] - Behold.
MYSELF (אנכי)[Pronoun] - Myself
SENDING (שלח)[Verb] - Will Send.
YALL (לכם)[Preposition(2nd pers.plur.masc.)] - To You All
(את) - The Direct-Object Marker in Biblical Hebrew
GOD YE (אליה)[Noun] - Elijah.
PROPHET (הנביא)[Noun]
TO FACE OF (לפני)[Noun] - To Face Of
COMING (בוא)[Verb] - To go or come
DAY (יום)[Noun]
GOD (יהוה)[Noun] - God's Name. He Will Exist.
GREAT (הגדול)[Adjective]
AND TERRIBLE (והנורא)[Verb]
The first word is a Particle. A Particle is: an adverb or preposition used with a verb to form a phrasal verb. For example in the sentence "He quickly put on his clothes," "on" is a particle.
So the first word connects to the third word like "See the Sending".
The Second word is a Pronoun. A Pronoun is: a word that can function by itself as a noun phrase and that refers either to the participants in the discourse (e.g., I, you ) or to someone or something mentioned elsewhere in the discourse (e.g., she, it, this ). So shows "What is sending." In this case "Sending Myself".
The Fourth word is a Preposition. A Preposition is: a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause. In this case the other word in relation is "MYSELF" proceeding the noun Elijah that is built from two Hebrew Words "אל" meaning God + "יה" meaning God's Name. Therefore through the meaning to the Hebrew word. And not the identity associated with the name "Not the actual person Elijah".
The fifth word Et is used to designate that the following word is the definite direct object of the verb. What Verb? Answer: "SENDING". What is the following word? It follows the word meaning "God God's Name".
So Decoded
Remember the Law of Moses, My servant, Which I commanded him in Horeb for all Israel, With the statutes and judgments. (Malachi 4:4 NKJV)
See I am sending to you all God, God's Name the Prophet, to face the coming day, God great and terrible. (Malachi 4:5 Decoded)
And he will turn The hearts of the fathers to the children, And the hearts of the children to their fathers, Lest I come and strike the earth with a curse.” (Malachi 4:6 NKJV)
Why does John the Baptist deny being "Elijah"?
Because he was not Elijah the man or the Prophet coming in Gods Name.
Look carefully at the response of Jesus.
But I say to you that Elijah has come already, and they did not know him but did to him whatever they wished. Likewise the Son of Man is also about to suffer at their hands.” (Matthew 17:12 NKJV)
The disciples come to their own conclusion
Then the disciples understood that He spoke to them of John the Baptist. (Matthew 17:13 NKJV)
My conjecture is that the conversation gets cut short because of the boy
And when they had come to the multitude, a man came to Him, kneeling down to Him and saying, 15 “Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water. (Matthew 17:14-15 NKJV)