MGS3 was very sad, and in the end it had that feeling of "hoplessness". What else could have drived Big Boss forward if not a life altering event like that one.
MGS4 was intensely emotional and overwhelming, but at the same time it was surprisingly "hopeful". All those incredibly iconic characters are saying a final good bye, but ultimatelly the world has a second chance because of them. It was something quite unique, sad and optimistic at the same time.
"You have the right to the remains of a silent attorney"