Advent IV is the fourth Sunday in the liturgical calendar as well as the fourth and last in the season of Advent.  Advent IV can come as late as December 24th.

The liturgical color is violet (or "purple").

All the candles are lit in the Advent wreath in the chancel.  

The setting of the mass is Plainsong for the Kyrie, Sanctus, Benedictus and Agnus Dei.  The Gloria is omitted in Advent.

The Proper of the mass is at DWM pg. 164.

The celebrant, rather than the deacon, reads the Prayers of the People using Form I (See DWM 1048).

The Preface is the Preface of Advent (DWM pg 582)