I tap two Forests and use Garruk Wildspeaker's +1 ability to untap them, if I retap them will I have 4 mana?

[Jon]

More specifically, say I have four Forests in play and Garruk. If I tap two Forests, use Garruk's +1 ability to untap them, and then tap all four Forests, would I be able to play something that costs (4)(G)(G) since I would have six in my mana pool?