Traditionally, the prayers used for the Advent wreath for each week of Advent are the collects, or short prayers at the beginning of Mass, for the Sunday of Advent that begins that week. The text given here is of the collect for the Fourth Sunday of Advent from the Traditional Latin Mass; you could also use the Opening Prayer for the Fourth Sunday of Advent from the current missal.
In this fourth and final week of Advent, we ask Christ to forgive us for our sins and, through His grace, to create us anew when He comes.
Advent Wreath Prayer for the Fourth Week of Advent
Bestir, O Lord, Thy might, we beseech Thee, and come; and with great power come to our aid, that, by the help of Thy grace, that which is hindered by our sins may be hastened by Thy merciful forgiveness. Who livest and reignest, with God the Father, in the unity of the Holy Ghost, God, world without end. Amen.