They pointed me to a Microsoft article (Article ID: 888575) on the subject which confirmed this and offered further help:
Microsoft Article ID: 888575 wrote:
You cannot put a computer that is running Windows XP, Windows Server 2003, Windows Vista, or Windows Server 2008 into hibernation. Additionally, the Hibernate tab in the Power Options Properties dialog box is unavailable.
NotesThis issue occurs if your computer has more than 4 gigabytes (GB) of random access memory (RAM).
This issue occurs in both 32-bit and 64-bit versions of Windows.
This issue occurs because hibernation is disabled on computers that have more than 4 GB of RAM.
Hibernation requires sufficient disk space to contain the contents of the computer's memory. Performance is poor on a computer that has more than 4 GB of memory and that has support for hibernation. Therefore, Microsoft has disabled support for hibernation on such computers.
To re-enable the hibernation option while (if) you are determined to keep your RAM level on 4 Gb or above, this is what is being suggested to do:
Microsoft Article ID: 888575 wrote:
Note Sometimes, the Hibernation tab is unavailable even after you decrease the RAM to 4 GB or less. This issue occurs because PCI memory resources are mapped in the middle of 4 GB physical memory address spaces, and the rest of the RAM is mapped to more than 4 GB of physical memory address space. If the value of the highest physical memory address is larger than 4 GB, Windows determines that the computer has more than 4 GB of memory. Therefore, the Hibernation tab is disabled. In this scenario, you must add the /MAXMEM=4096switch to the Boot.ini file to reduce the highest physical memory to 4 GB.
For more information about the MAXMEM switch, click the following article number to view the article in the Microsoft Knowledge Base:
108393 The /maxmem switch in the Windows Boot.ini file: http://support.microsoft.com/kb/108393/
To work around this issue on a computer that is running Windows Vista, enable hibernation. To do this, run the following command as an administrator:
powercfg /h on
To do this, follow these steps:
1-Log on as an administrator.
2-Click Start, type cmd in the Start Search text box, right-click cmd.exe, click Run as administrator, and then click Continue.
3-At the command prompt, type powercfg –h on, and then press ENTER.
To view the Power Options Properties dialog box, click Start, click Run, type powercfg.cpl, and then click OK.
This is the full article on this matter:
http://support.microsoft.com/kb/888575
Hope it helps to those who want to keep their +4 Gb RAM and also the hibernation mode intact.