Is it possible to connect more than 4 Wireless Xbox 360 controllers to a PC?

Yes you can use more than 4 wireless controllers on some games. For example Gang Beasts, Knight Squad, Miner Warfare and Screencheat support up to 8 controllers via DirectInput instead of XInput.

XInput supports a maximum of four controllers at a time. This is an Xbox limit, carried over to Windows. Although as of 2010 few PC games require more than four controllers at once, DirectInput itself has no such limitation.

(Wikipedia, DirectInput vs. XInput)

DirectInput works with any controller including Xbox controllers. The use of DirectInput or Xinput is down to the developer or engine that they are using. It is not something that you can configure in Windows.

Update: I noticed that the upcoming windows wireless adapter states that it "(s)upports up to 8 wireless controllers". I wonder if they've removed the XInput limit and whether this will be for windows 10 only or if they will eventually backport to Windows 7.

You can connect them, and all the controllers will show up as devices on your PC, but you won't actually be able to use more than 4 controllers.

XInput (the API that Windows games use to interface with Xbox 360 controllers) only supports a maximum of 4 controllers. Even if there were a way around this, games are designed with this limitation in mind.

I just got 7 xbox controllers to work perfectly on Win 10. The setup was crazy: 2 dongles plugged to the PC, three wireless controllers, two cabled controllers plugged to the PC and other two other cabled ones plugged to another PC (for lack of USB slots on this one), but all of them working on this PC.