I play lots of minecraft, and yes it's an extremely laggy game, but there must be a way to make it so that it uses 3 out of 4 Gb for minecraft/graphics and less for other stuff. Please give a fairly easy walkthough.
If Minecraft lags on your PC, you have bigger issues than how to allocate your RAM. Minecraft is very easy on PCs to run, if you cant run it, chances are your hardware is older than I am, and im 42.