That 5 could also be an S ( MGS4:S....?) I'm 90% positive we are talking about an MGS: Raiden though, possibly to tell the story of Sunny's rescue or (more likely) something in the future of MGS4 events.
They could even cover both Sunny's rescue (a playable memory) and tie it to events post MGS4, this i think would be the best approach. But hell, whatever Kojima is cooking, he always ends up surpassing my expectations, no worries here.
Oh, the anticipation !!! ^_^
Raiden and Sunny would be likely with the teaser website. Raiden means lightning and the weather is set to change to Sunny (in the flash files) and then to an "exclamation mark". So yeah, I think that's the best guess - or it's what Kojima wants us to think