Here are a few clues on how to unlock modes and weapons from “Call of Duty 4: Modern Warfare” on the Xbox 360. Unlockable: Prestige Mode To unlock Prestige Mode, which is a multiplayer-only mode, you must reach experience level fifty-five. Instead of capping out at level fifty-five, which is the highest level, Prestige Mode allows you to start again from level one. This can be done a total of ten times, giving you ten different icons in in-game lobbies to show your extreme achievement. Unlockable: Arcade Mode To unlock Arcade Mode, beat the game once through on any difficulty level. Arcade Mode itself is split into two options –…