The essential difference between 2-line and 4-line kites is the fact that in a 2-line kite you have no "bar depower" - ie the ability to decrease the power in the kite by pushing the bar away from you. read more
Ie you can move the kite to the zenith or to the edge of the window to minimize power. Of course this increases the possibility that you might be lifted into the air in high winds. In a 4 line kite you can push the bar away from you and your kite will immediately lose power. Kiteboarding started with 2 line kites. read more
It really depends on you preferences and the conditions you ride in. read more