The four-fold use of the phrase "I will raise him up on the last day" is bookended by two important ideas:
1.) Of all that the Father had given Jesus, Jesus would not suffer the loss of any, because it was the Father's will that Jesus not lose any.
John 6:39 (ESV),
39 And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day...
Not the emboldened phrase"I should lose" above. It is the Greek word ἀπολέσω (apolesō), which is the aorist subjunctive active, first person singular of the Greek verb ἀπόλλυμι (apollumi).
It means to perish/perish completely or destroy/destroy utterly.
2.) The chapter ends with a reference to Judas Iscariot:
John 6:70-71 (ESV)
70 Jesus answered them, “Did I not choose you, the twelve? And yet one of you is a devil.” 71 He spoke of Judas the son of Simon Iscariot, for he, one of the twelve, was going to betray him.
We know from a later text in the Gospel of John that Judas was a thief:
John 12:4-6 (ESV),
4 But Judas Iscariot, one of his disciples (he who was about to betray him), said, 5 “Why was this ointment not sold for three hundred denarii and given to the poor?” 6 He said this, not because he cared about the poor, but because he was a thief, and having charge of the moneybag he used to help himself to what was put into it.
Further, in between what Jesus says in John 6 and what we learn of Judas in John 12, we have in John 10 the following idea:
John 10:10 (ESV)
10 The thief comes only to steal and kill and destroy. I came that they may have life and have it abundantly.
Note the use of destroy above. Here, it is ἀπολέσῃ (apolesē), the third personal singular aorist subjunctive active of ἀπόλλυμι (apollumi), meaning the same things as from John 6:39.
Jesus, by indicating four times that He will raise what the Father committed to His trust on the last day, not losing any, in conjunction with the reference to Judas as a devil, that is, a thief whose only interest is in stealing, killing, and destroying, is making a reference to the Mosaic law of a four-fold restitution whenever a theft has taken place, particularly for a sheep (See: Exodus 22:1, 2 Samuel 12:6, and Luke 19:8).
It is important here to distinguish the fact that the four-fold restitution is specifically for sheep.
First, let it be noted that a common comparison of humans falling into sin is with sheep going astray (See, e.g.: Psalm 119:176, Isaiah 53:6, and Jeremiah 50:6).
Second, note that in John 10:11 Jesus tells His audience that He is the "good shepherd" of the sheep. He goes on to say He will lay down His life in order to rescue the sheep of His sheepfold whenever either thieves and robbers come up, or when wolves are on the prowl.
This surrendering of the life of the good shepherd is the means whereby Jesus procures the restoration of the lost sheep who have gone astray, so that they may have eternal life and be saved. It is how He fulfills the will of the Father and makes certain that, on the last day, none of them are lost.
Jesus emphasizing that fact four times within just a matter of a few verses in John 6 then indicates that He will restore four-fold that which has been taken away from the Father, that is, Jesus will make good on what the Thief, that is, the Devil, has stolen, namely, the human race.
