Microsoft confirms that user cannot put a computer that is running Windows XP, Windows Server 2003, Windows Vista, or Windows Server 2008 with more than 4 GB (gigabytes) of RAM (random access memory) installed into hibernation mode. This is not a bug but by design, as Microsoft disable hibernation support on PC with more than 4GB physical memory inserted because benchmark by Microsoft shows that performance is poor on a computer that has more than 4 GB of memory and that has support for hibernation, which requires sufficient disk space to contain the contents of the computer’s memory.

Source: Thinkdigit