But what about 4th Edition?
Without reading the whole topic (that I'm going to do in the meantime), so I don't know if the answer has already being given, but yes, Greyhawk exist(ed) in 4e. It is considered to be an "alternative world" to the Nentir Vale's world, from where some places and some NPCs came from.
For instance, Inverness Castle its said to have originated in another world, and it exist simultaneously in two worlds at the same time, one of these worlds being the world of the Nentir Vale. The other is heavily implied to be Greyhawk. There where other places from Greyhawk also connected to the Nentir Vale, like the Underdark city of Erelhei-Cinlu. While not explicitly stated lorewise, this can be explained as an after effect of the Dawn War. The violent battles between gods and primordials destroyed the planar boundaries in some places, and in these places the other dimensions "bleed" into the world of the Nentir Vale.
Likewise, some NPCs like Mordenkainen, Evard, and Emirikol, had travelled to the Nentir Vale world at least once. For instance, Evard has a manor in the small village of Duponde, in the Nentir Vale, while
Mordenkainen's Magnificent Emporium is an in-universe book where Mordenkainen catalogued a lot of magic items and artifacts created by the ancient empires of the world of the Nentir Vale (Nerath, Arkhosia, and Bael Turath).
Also, the Vecna of the Dawn War pantheon is specifically said to be the same Vecna from the Flanaess.
There are also a few Greyhawk adventures published in 4e, like "The Last Breath of Ashenport" or "The Battle of Emmiry(sp?) Meadows". And a few others that mix Nentir Vale with Greyhawk elements, such as the Against the Giants series.