Convert 4-band indexed image to digital numbers

Hi All,

I have a 4-band image (including Red,Green,Blue and NIR bands) and would like to access each band of the image with the digital numbers to be within 0-255. The problem is that each band of the image is an indexed image so when I use ind2gray (g1=ind2gray(RGBN(:,:,1),gray)), the values all become 65535 for g1, which isn't obviously correct. If I use g1=ind2rgb(RGBN(:,:,1),gray)), then g1 become an RGB image alone which isn't what I want. Indeed, each band is a single intensity image and the combination of bands (like R G and B) will become true-color image.

Does anybody know how I can access each band and change its values to be within 0-255? Here is some info about the image using imfinfo command:

info =

                     Filename: RGBN
                  FileModDate: '25-Jun-2014 16:06:39'
                     FileSize: 31712267
                       Format: 'tif'
                FormatVersion: []
                        Width: 1796
                       Height: 1654
                     BitDepth: 64
                    ColorType: 'grayscale'
              FormatSignature: [73 73 42 0]
                    ByteOrder: 'little-endian'
               NewSubFileType: 0
                BitsPerSample: [16 16 16 16]
                  Compression: 'LZW'
    PhotometricInterpretation: 'BlackIsZero'
                 StripOffsets: []
              SamplesPerPixel: 4
                 RowsPerStrip: 4.2950e+09
              StripByteCounts: []
                  XResolution: []
                  YResolution: []
               ResolutionUnit: 'None'
                     Colormap: []
          PlanarConfiguration: 'Chunky'
                    TileWidth: 128
                   TileLength: 128
                  TileOffsets: [195x1 double]
               TileByteCounts: [195x1 double]
                  Orientation: 1
                    FillOrder: 1
             GrayResponseUnit: 0.0100
               MaxSampleValue: [65535 65535 65535 65535]
               MinSampleValue: [0 0 0 0]
                 Thresholding: 1
                       Offset: 8
                    Predictor: 'None'
                 ExtraSamples: [3x1 double]
                 SampleFormat: {4x1 cell}
           ModelPixelScaleTag: [3x1 double]
             ModelTiepointTag: [6x1 double]
                GDAL_METADATA: '<GDALMetadata>
  <Item name="DataType">Generic</Item>
</GDALMetadata>
'
                  GDAL_NODATA: '65536'

Thanks, Mehdi