I remember a colleague who bought an additional ram for their home computer. When they had a gathering in his home, one of the techs who were there checked their computer and found out that the additional RAM were not being used. I don't exactly remember the problem on why that extra memory was not utilized but it's something about the OS. Is anyone here familiar with that?
It sounds like the operating system was a 32 bit version of the OS when the computer and it's hardware were capable of running a 64 bit system. Although other operating systems can use ram past the 4GB limit by using an overlay to reallocate segments beyond protected mode in newer processors (feature wasn't always available but when it was, linux and BSD used it), Windows is still limited because they do not and decided not to so that they could keep the 32 and 64 bit sales alive longer.
With the Windows GUI over its modified dos-based operating system, you were/are limited to 4GB of ram on 32 bit OSes. The system can usually only give about 3.75GB of ram out of 4GB if you're using a 32 bit system, because it uses 256MB of that ram for itself just to run.
With Windows 64-bit systems, the address space can be reached without any bank switching or overlays to get to that ram, and the hardware is able to see and use it all right away. So if you're running a 64 bit version of Windows, you'll be able to see and use all of the RAM available on the PC. If you're using a 32 bit version of Windows, it'll only see about 3.75GB of it.
It is possible to implement an overlay for protected mode switching to get to it even on a 32 bit version of Windows (it's a little like DPMI for DOS going back about 20 years, which was a tool that let you switch modes to give DOS itself the capability to use up to 4GB of ram through bank switching, and let you switch between real mode to protected mode and back for programs that needed more than 16MB at a time per frame). It worked well (and still does if you use DOS). There is a 32 bit version of FreeDOS which has extended that since.
This is probably more than you wanted to know lol...the summary and quick answer is that they were probably running a 32 bit version of Windows and couldn't see all of the ram they had or use it until the assistant they had replaced the OS with a 64 bit version that could for them.