Siliconera points us in the direction of "Metal Gear Solid Existence." It's a name Konami filed in the Japanese Trademark database. We can't say for sure what this title will be used for -- is this some sort of new game altogether or is it the name of an MGS4 re-release a la Substance and Subsistence? It certainly seems to fit the shoe of the latter category.
However, all we have to go with right now is a name; there's not enough facts and evidence to say for sure. It might not ever be used. So for now, we're filing this under 'rumors.' Perhaps with Tokyo Game Show just around the bend, we might hear of something official soon.