Available only on Magic Online, Masters Edition IV features 269 cards with randomly inserted premium cards and goes on sale for a limited time beginning January 10, 2011. Each Masters Edition IV booster pack contains 15 randomly sorted cards (1 rare, 3 uncommon, 10 common, and 1 of the following lands: Urza's Mine, Urza's Power Plant, or Urza's Tower.) and is available for $3.99 USD in the Magic Online store.
Masters Edition IV includes 12 land, 80 common, 72 uncommon, 105 rare cards and is not redeemable.
The set goes on sale in Magic Online beginning January 10 and will be available for a limited time.
Masters Edition IV cards are legal in all Classic-based Constructed formats (unless otherwise noted in the format banned and restricted lists): Classic, Pauper, Singleton, Tribal Wars (Classic), Classic with Vanguard, Prismatic, and Freeform.