Recent reports show that FromSoftware has supposedly cancelled the XBox 360 version of Armored Core 4 and the game is now solely for the PlayStation 3, based on translating an announcement on their Japanese site. This was to be the first time the series has been on another system other than the PlayStation line.
Two reasons allegedly given by CEO Naotoshi Zin are a new contract signed between themselves and Sony to make the game exclusive (which also paves the way for an animated movie based on the game by Sony Entertainment) and supposedly the 360 version would be "too complex to develop". This second reason doesn't seem quite right as they have had a 360 development kit for almost a year and From also worked along with Sega to produce the game Chrome Hounds for the XBox 360.
Once this is confirmed as fact (or dismissed as yet another false rumor) an update will be posted.