Although a system may accept 4GB, much if not all of the top GB (and possible some of the GB between 2 and 3) is using addresses that are already taken by components of the system (such as the video card) and so it cannot be used. You would be wise to regard 3GB as the limit for a 32 bit system, unless you are expecting to dual boot with, or imminently upgrade to a 64 bit system.