It has not yet been introduced, and is certainly not yet on sale, but someone supposedly has an Apple iPad 4.  

Over Christmas vacation, a burglar broke into a Microspft campus in California and swiped a pile of iPads, leaving the Microsoft products untouched.  But according to a police report on the stolen products, the thief took off with 2 iPad 2's, 2 iPad 3's,and what is described as an iPad 4.

Microsoft uses that building to develop software for Apple products, which explains the presence of the iPads.