Zanzibar Land
The target location in Metal Gear 2: Solid Snake, Zanzibar Land is situated between Russia, Afghanistan, Pakistan and China. You can actually visit Zanzibar Land by picking up a copy of MGS3: Subsistence, which bundles an updated version of the MSX2 classic.
If Outer Heaven is making an appearance, we're confident that Zanzibar Land could too. Interestingly, in MG2, Zanzibar contains a production factory making smaller, non-nuclear Metal Gears - early prototypes of the Gekkos? This is our best bet for the fifth location.
Shadow Moses Island
A nuclear weapons disposal factory and the locale for Metal Gear Solid. We already know that Metal Gear REX (the version held at Shadow Moses) will appear in MGS4 to face off with Metal Gear RAY (from MGS2), so a return to Shadow Moses isn't out of the question.
Galuade Fortress
Galuade was actually located inside Zanzibar Land, as you'll know if you've played the GameBoy Color edition of Metal Gear Solid. However, since this version isn't part of the series' main story arc, Galuade Fortress probably won't make it into MGS4, though something resembling it might still appear.
Lobito Island
In a similar vein to Galuade, Lobito - the location for Metal Gear Acid - isn't part of the MGS 'canon' and so isn't as likely to turn up in the new game. But, given Hideo Kojima's disregard for normal storytelling methods, we wouldn't write anything off simply because of logic.
The United States
With the 'Guns of the Patriots' subtitle, it's clear that MGS4 will explain more about the Patriots. Or, at least, throw in more bizarre and confusing detail about these artificial intelligences who actually run the United States, and who back the global uprising led by Liquid Ocelot. So, surely, a return to the US (after Raiden's climactic battle atop Federal Hall with Solidus Snake ) is essential? Unless the Patriots are actually based somewhere else completely...
