now it could easily sell a million units which i think would be enough to recoup the cost no
It doesn't really matter if it could easily sell a million units, and recoup the cost. This is a place of business, its not some cult with an alternative motive to spread art and joy and influence to the world through games.
This is how a project assessment of MGS4 would be:
A: Predict potential income, discounted on probability (which gives you a fairly good number to work with, basically what you do is have multiple "scenarios" then calculate probability for that scenario, multiply probability in each scenario with cash flow, and add it all up).
B: Talk to Sony about getting subsidies (i.e. monetary grant) if possible.
C: Predict cash flow from a possible port.
D: Talk to MS about getting subsidies for porting it.
E: Add potentia loss by hurting Kojima's feelings (if there is any). For example, will Kojima quit, if we do this? How much is his name worth etc..
If (A)+(B) < (C)+(D)-(E) there is going to be a port.
If you can recoup the cost just making a ps3 version is irrelevant, if you can earn more money by porting it. In the end, the stockholder want return on their investment, they don't care about how good MGS4 might look and play on a PS3.