The problem is with 64-bit windows. You need to reboot it with boot options to allow installing and using unsigned device drivers. It is MUCH easier to do it on 32-bit windows (or in a 32-bit windows Virtual Machine running insted 64-bit windows or linux).
Feel free to write a "how to revert a K4 to 4.0.0 guide", and please post it back to the forum (and index wikis). Thanks.