Why 2.66 GB usable in 4 GB ram? [duplicate]

I am having windows 10 pro 64 bit OS,With 4 Gb RAM. But in memory section its showing 2.66 GB usable. Why? doing some research(Googling) I found that 32 bit machine can't utilize the full RAM. But though I am having 64 bit machine, still why its showing 2.66 GB usable? I tried a tweak through MSConfig → Boot → Advanced option → unchecking but nothing is working.

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System Information
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      Time of this report: 6/3/2016, 08:39:50
         Operating System: Windows 10 Pro 64-bit (10.0, Build 10586) (10586.th2_release.160104-1513)
                 Language: English (Regional Setting: English)
      System Manufacturer: LENOVO
             System Model: 20351
                     BIOS: InsydeH2O Version 03.73.069ACN28WW
                Processor: Intel(R) Core(TM) i3-4030U CPU @ 1.90GHz (4 CPUs), ~1.9GHz
                   Memory: 4096MB RAM
      Available OS Memory: 2722MB RAM
                Page File: 3268MB used, 1885MB available
              Windows Dir: C:\WINDOWS
          DirectX Version: 12
      DX Setup Parameters: Not found
         User DPI Setting: Using System DPI
       System DPI Setting: 96 DPI (100 percent)
          DWM DPI Scaling: Disabled
                 Miracast: Available, with HDCP
Microsoft Graphics Hybrid: Not Supported
           DxDiag Version: 10.00.10586.0000 64bit Unicode


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Display Devices
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          Card name: Intel(R) HD Graphics Family
       Manufacturer: Intel Corporation
          Chip type: Intel(R) HD Graphics Family
           DAC type: Internal
        Device Type: Full Device
         Device Key: Enum\PCI\VEN_8086&DEV_0A16&SUBSYS_380C17AA&REV_0B
     Display Memory: 1473 MB
   Dedicated Memory: 112 MB
      Shared Memory: 1361 MB
       Current Mode: 1366 x 768 (32 bit) (60Hz)

This is to do with Memory Mapped I/O.

Your computer will map areas of memory in order to access other devices quickly. In your case, your display is mapped to 1473 MB which means, yes, you do have 4GB of RAM installed however 1473 MB is reserved for Display operations. Normally, on-board graphics will take a larger slice of your RAM than a dedicated card however its not just display that can take out a chunk of RAM. The memory which isn't mapped is your available; as in available to the OS solely.

If you would like to look into this further check this out Memory Mapped I/O