Story changes:
"METAL GEAR SOLID: RISING was a spin-off intended to feature the actions and transformation of Raiden, one of METAL GEAR SOLID 2: SONS OF LIBERTY’s main characters, in the lead-up to METAL GEAR SOLID 4: GUNS OF THE PATRIOTS.
However, after Platinum Games came on board, the story was changed to take place several years after METAL GEAR SOLID 4 to give them more of an opportunity to show off what they can bring to the series. Cyborgs are spreading at a rapid pace in the game’s world, resulting in earth-shattering superhuman action at every turn!
The original aim for a combination of stealth and action has also been enhanced, for a more aggressive new focus on pure, exhilarating action."
http://www.konami.jp/mgr/en_us/index.html
I approve of this.
They usually end up contradicting the previous canon with prequels so I like that they set it in the future after MGS4.