No, it REALLY doesn't.
All it confirms is that it IS a 5, with clocks 4-1 counting down to E3. Microsoft isn't the only group at E3. We could be seeing MGS4Subsistencey for PC, MGS4+Oxide for 360 and PC, Snatcher, Zone of the Enders or some new IP.
And trust me, I'm 99% sure this announcement is about a PC release.
KojiPro spent a lot of time developing an engine for the PS3. Porting it to the 360 would mean scaling down. It'd mean multiple discs and possibly hardrive installs.
But, on the PC... things are different. Very different. The same limits don't exist. Take a look at the Source engine, constantly upgraded and reused, highly optimised... Having a powerful PC engine would be a massive benefit to Konami. We could be seeing something massive.
Remember: Metal Gear Solid 1 and 2 went to PC. MGS4 is not a contract exclusive. It's all to do with the power of the platform. And the PS3 and PC are almost equals.
Well, many PS3 titles require HDD installs nowadays while the Xbox 360 counterpart does not. For some reason developers have been reluctant to reveal why these installs are mandatory: I still don't think there is a concrete answer out there. So if MGS4 made it to 360, I'm willing to bet it would not require an HDD.
MGS4 didn't fill up a full Blu-ray from what I heard and a lot of space was sort of wasted on uncompressed audio. Things like that could be sacrificed for the 360 version and no one would notice a difference.
I still think the announcement is Microsoft centric if it's happening on June 1. They are the only company holding a press conference that day. Sony's presser isn't until the next day. I really don't see Konami bothering with PC so that's why I said Xbox 360 but we'll see.