The memory isn't "wasted". You seem to be stuck on the thought that the only memory your computer has is contained in the DIMMS. This is incorrect.
When you install 4GB worth of memory sticks in the slots on your motherboard, you need to understand that is In Addition To a small amount that's built into your Mobo, plus however much is in your Video Card. Devices also take up some addresses, and anything with a cache needs to have that cache addressed as well.
So: Mobo has a few meg built into it, your video card has 250GB~1000GB, your processor has few MB cache, plus a few hundred more Kb for your device addresses, including the 8/26/32MB cache in your hard drive and that sort of thing. After all that has been done, then comes the 4GB worth of DIMMS you installed on your Motherboard. ALL of these need to be fully addressed so your Operating System, drivers, and applications can communicate to them. But the total in this case is much more than 4GB.
So what happens is your Mobo claims the first addresses, then the cache on your CPU, then the cache on your HDD, then however much your other devices need, then however many addresses your video card needs. All of this comes out of the 4GB total. WHAT IS LEFT OVER after the rest of your comp is handled can be used for your installed DIMMS. In this case there is 4GB worth of DIMMS, but a large chunk of addresses have already been assigned to other purposes. So the system assigns however many addresses remain. Again: 4GB worth of potential addresses, less Mobo, Devices, Caches, and Video memory. Whatever is left over is used for your system memory. And that is how you get to 3GB~ish displayed memory in your OS.
In the case of 2GB of RAM, your system still assigns from the same 4GB pool of addresses. So you have the 1GB~ish total for your system, followed by the 2GB DIMMS. The total is less than 4GB, therefore you will see all of the RAM.