I had 6 GB RAM, now Windows is telling me I have only 2 GB. Where's the other 4 gone?

The first step in figuring this out is to actually open the computer and see if it's there.

You can physically remove it and put it back in by following this Youtube video It might not be your exact model but it should still work.

Once you have confirmed that you physically have the RAM and it is seated properly then boot up your machine and go into BIOS by pressing F2 or F10 or whatever it may be on your laptop.

Find the memory section and make sure it is reporting all of your 6GB

Now boot into Windows and see if your RAM is properly showing by checking either the task manager (Ctrl+Shift+Esc) or right-click on Computer and go to properties.

If you do not have the correct amount showing then shut down again and physically remove all of your RAM. You will probably have a 4gb stick and a 2gb stick so put only one stick into slot 1 and start up your machine, does the RAM get reported?

Power down again put in a different stick all by itself, does that one get reported?

If your computer simply fails to boot and probably will make a beeping noise then you know that you in fact have a bad stick of RAM.

It could also be a case of having a bad RAM slot so individually test both slots with both sticks of RAM and see which combos either work properly or break.

There are many diagnostic things and methods you can attempt but this is probably the simplest and will give you the correct results.

Run Msinfo32 to display how much ram you have. It should be shown as the installed physical memory. If it isn't there, You'll need to do a memory test from your BIOS.