With those four cards you can turn 1 kill. Magic typically starts with each player at 20 life. Black lotus adds three green mana. Channel costs two, but lets you pay one life to add one generic mana as many times as you want. Mox ruby pays for the red in fireball. Pay 19 life for 19 mana, with the last green you have 20 extra mana to invest into fireball, dealing 20 damage to your opponent and ending the game.