It is unlikely that MGS4 will be ported to another system, but it would not really surprise me. They have no legal obligation to keep the series on one system and have ventured onto other consoles before. Even if MGS4 is a bundle-title, it’s not like Konami can be sued for putting it on the 360 later. Also, I wouldn’t exactly call Square-Enix a flagship for Sony.
Here’s a fun way to remember it:
First-Party=Monarch
Second-Party=Concubine
Third-Party=Street *edited*