getting a mean from a 4d matrix

I have a file with the dimensions below. I would like to plot pcolor plot of the area for the average CHL over a period of a few months. does anyone know how to do this? Dimensions: [1x4 struct] Variables: [1x7 struct] Attributes: [1x15 struct] Groups: [] Format: 'classic'

Source:
           D:\Dissertation\Primary production data\dataset-global-nahindcast-bio-001-018-V5-chl_1521044729974.nc
Format:
           classic
Global Attributes:
           title           = 'monthly mean chorophyll from Global Biogeochemical hindcast'
           institution     = ' '
           references      = 'http://www.mercator-ocean.fr'
           source          = ' '
           Conventions     = 'CF-1.0'
           history         = 'Data extracted from dataset http://opendap-ran.mercator-ocean.fr:8080/thredds/dodsC/dataset-global-nahindcast-bio-001-018-V5-chl'
           time_min        = 3598862400
           time_max        = 3659256000
           julian_day_unit = 'seconds since 1900-01-01 00:00:00'
           z_min           = 30.8746
           z_max           = 77.6112
           latitude_min    = -12.8899
           latitude_max    = 12.8899
           longitude_min   = 55
           longitude_max   = 90
Dimensions:
           x            = 143
           y            = 105
           time_counter = 24
           deptht       = 8
Variables:
    deptht      
           Size:       8x1
           Dimensions: deptht
           Datatype:   single
           Attributes:
                       long_name              = 'Depth'
                       units                  = 'm'
                       positive               = 'down'
                       unit_long              = 'Meters'
                       standard_name          = 'depth'
                       _CoordinateAxisType    = 'Height'
                       _CoordinateZisPositive = 'down'
                       valid_min              = 30.8746
                       valid_max              = 77.6112
                       axis                   = 'Z'
    time_counter
           Size:       24x1
           Dimensions: time_counter
           Datatype:   double
           Attributes:
                       standard_name       = 'time'
                       long_name           = 'Time axis'
                       calendar            = 'gregorian'
                       units               = 'seconds since 1900-01-01 00:00:00'
                       time_origin         = '1900-01-01 00:00:00'
                       bounds              = 'time_counter_bounds'
                       unit_long           = 'seconds since 1900-01-01 00:00:00'
                       _CoordinateAxisType = 'Time'
                       valid_min           = 3598862400
                       valid_max           = 3659256000
                       axis                = 'T'
    CHL         
           Size:       143x105x8x24
           Dimensions: x,y,deptht,time_counter
           Datatype:   single
           Attributes:
                       _CoordinateAxes  = 'nav_lon nav_lat deptht time_counter deptht y x '
                       units            = 'mg.m-3'
                       missing_value    = 9.969209968386869e+36
                       long_name        = 'Mass Concentration of Chlorophyll in Sea Water'
                       short_name       = 'CHL'
                       standard_name    = 'mass_concentration_of_chlorophyll_in_sea_water'
                       coordinates      = 'nav_lon nav_lat deptht'
                       online_operation = 'average'
                       scale_factor     = 1
                       add_offset       = 0
                       savelog10        = 0
                       _FillValue       = 9.969209968386869e+36
                       unit_long        = 'milligram of chlorophyll per cubic meter'
    nav_lon     
           Size:       143x105
           Dimensions: x,y
           Datatype:   single
           Attributes:
                       _CoordinateAxes     = 'y x '
                       standard_name       = 'longitude'
                       long_name           = 'Longitude'
                       units               = 'degrees_east'
                       nav_model           = 'grid_T'
                       add_offset          = 0
                       scale_factor        = 1
                       unit_long           = 'Degrees East'
                       _CoordinateAxisType = 'Lon'
                       valid_min           = 55
                       valid_max           = 90
                       axis                = 'X'
    y           
           Size:       105x1
           Dimensions: y
           Datatype:   int32
           Attributes:
                       standard_name       = 'projection_y_coordinate'
                       units               = '1'
                       _CoordinateAxisType = 'GeoY'
    nav_lat     
           Size:       143x105
           Dimensions: x,y
           Datatype:   single
           Attributes:
                       _CoordinateAxes     = 'y x '
                       standard_name       = 'latitude'
                       long_name           = 'Latitude'
                       units               = 'degrees_north'
                       nav_model           = 'grid_T'
                       add_offset          = 0
                       scale_factor        = 1
                       unit_long           = 'Degrees North'
                       _CoordinateAxisType = 'Lat'
                       valid_min           = -12.8899
                       valid_max           = 12.8899
                       axis                = 'Y'
    x           
           Size:       143x1
           Dimensions: x
           Datatype:   int32
           Attributes:
                       standard_name       = 'projection_x_coordinate'
                       units               = '1'
                       _CoordinateAxisType = 'GeoX'