Windows cannot detect installed RAM if the size of the RAM is equal to or greater than 4 GB. This is not a RAM detection problem caused by faulty hardware or software . If the system is Windows XP or Vista 32 bit, there is a mathematical limitation to the size of the RAM that can be detected. Methods such as Physical Address Extension can be used to increase the size of the RAM but it may not work well for the general purpose PC. It is a known problem that Windows cannot detect installed RAM which is greater than 4 GB in size.
Due to mathematical constraints, Windows XP or Vista 32-bit edition cannot use 4 GB of RAM.
A 32-bit system can handle a maximum of between 2.5GB and 3.5GB of RAM.
This problem is not the result of a fault with your PC.
What can be done?
The Physical Address Extension (PAE) trick might solve the problem but it is not recommended and disabled by default by Windows. It can prevent some drivers from failing to load properly.
The recommended solution is to switch to a 64-bit operating system. In that case it might be worth buying a high-end machine with the 64-bit operating system pre-installed to ensure full driver compatibility.