The winner takes the last pebble!

There are $X$ pebbles in a pocket ( $X$ is a positive integer). $Adam$ and $Bob$ decided to play a game. Each turn, a player has to take either 1,2 or 3 pebbles out of the pocket. The one who takes the last pebble out wins the game.

Adam goes first.

Assume the two boys always follow an optimized strategy that has the best chance to win the game.

What is the condition of $X$ so that Bob wins the game?