Kojima said in a previous interview that MGS4's engine is specifically made for PS3. If they were to port the game over to the 360, it would basically be like remaking the game from scratch.
Both games have been confirmed they are coming to the PS3, and the 360. Remember we haven't seen the 3rd step in the Transfarring lifestyle yet.
I just think that Ground Zeroes is gonna be a short prologue. Remember in the GZ trailer when the burned man said "The trojan horse is in", or something like that. Why should it take several missions before the attack happens? Remember that the HD Collection PS3 edition held the full Metal Gear Solid 2: Sons of Liberty(with most Substance features), Metal Gear Solid 3: Snake Eater(with the Demo Theatre, 8bit games(MSX), and most of the extra camos/facepaint), and the full game of Metal Gear Solid: Peace Walker all remastered in HD.
I would say that they are gonna be released together as one package. It just seems silly to say that they both comprise of MGS 5. Then require buying 2 games to get the full story.
Plus according to metalgearsolid.com, Ground Zeroes is coming to the current generation consoles.
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