One of my favorites from the X series.
The main story of the game is that a Maverick destroyed the space lagoon which crashed and destroyed an entire city leaving only one person alive. The Maverick Hunters believe that the Repliforce were the ones that did it. On Zero's path you rescue Iris, a reploid who's brother is the colonel of the Repliforce. On X's path you meet Double ( which suits him well ) who informs you of the current situation regarding the Repliforce which are to take them to custody by any means necessary. Some of the enemies you fight are more fond of Zero than X such as Magma Dragoon. The gameplay its self is about the same as every other sidescroller. You go from one area into the next while defeating a group of enemies along the way while gaining power-ups. There are a total of 17 combined special attacks that X and Zero posses. X's path is more easier but Zero's path is more challanging. X uses his mega buster and Zero uses his sword; X is played better at long range but Zero is primarly a short range character. One of the biggest differences is that X's ultimate armor makes the hardly challanging even against the 3rd form Sigma ( the boss of the game )
Overall I would say the game seems to be a lot more about Zero and there is a good reason for that which is pretty obvious due the ending that was in X5. What X4 did compared to other sidescrollers to make the game feel less linear by having two playable characters and two different endings. X4 gave the series a new life even though the gameplay feels like same as previous X games.