Every single entry in the Metal Gear Solid series (barring the first) has been Hideo Kojima's "last" MGS. He claims he'll step away from the series every time but it seems like a compulsion for him -- like kleptomania, only with longer cutscenes.
It really gives the impression that he can't help himself, especially since he's already dreamed up three concepts for Metal Gear Solid 5 but needs his team to help him not make them. He's hoping that someone in Kojima Productions will have a better idea than him so he doesn't have to work on MGS5.
Oh Kojima-San, you are absolutely insane.
While Hideo is like a messiah of gaming to me, I wouldn't mind him taking a step back from Metal Gear. I'd like to see him have the time to create something brand new. Kojima notes that the MGS4 team is returning from their vacation, so the next Metal Gear will likely be decided soon. Somehow, I can't help but feel that Kojima will be as involved as ever.