Dan, I have tried it but it doesn't work with Reresh TOC. Below is the complete script.
import arcpy
from arcpy import env
import arcpy.mapping
# Set environment settings
env.workspace = "C:\Users\Documents\NGN\Regions"
arcpy.env.overwriteOutput = True
#define projection
Prjfile = r"C:\Users\Documents\NGN\Regions\WGS1984.prj"
spRef = arcpy.SpatialReference(Prjfile)
#Local Variables
FC3 = r"C:\Users\Documents\NGN\Regions\South_DO.gdb\MW_Decision"
FC4 = r"C:\Users\Documents\NGN\Regions\South_DO.gdb\RF_Decision"
mxd = arcpy.mapping.MapDocument(r"C:\Users\Documents\NGN\DO Maps\Demo_South.mxd")
df = arcpy.mapping.ListDataFrames(mxd,"Layers")[0]
arcpy.MakeFeatureLayer_management(FC4, r"C:\Users\Documents\NGN\Regions\RF_Decision.lyr")
layer4 = arcpy.mapping.Layer(r"C:\Users\Documents\NGN\Regions\RF_Decision.lyr")
Sym_layer4 = r"C:\Users\Documents\NGN\Symbology layer new\RF_Decision.lyr"
arcpy.ApplySymbologyFromLayer_management (layer4, Sym_layer4)
arcpy.mapping.AddLayer(df, layer4, "TOP")
lyr4 = arcpy.mapping.ListLayers(mxd, "", df)[0]
lyr4.visible = False
lyr4.name = "RF_Decision"
arcpy.MakeFeatureLayer_management(FC3, r"C:\Users\Documents\NGN\Regions\MW_Decision.lyr")
layer3 = arcpy.mapping.Layer(r"C:\Users\Documents\NGN\Regions\MW_Decision.lyr")
Sym_layer3 = r"C:\Users\Documents\NGN\Symbology layer new\MW_Decision.lyr"
arcpy.ApplySymbologyFromLayer_management (layer3, Sym_layer3)
arcpy.mapping.AddLayer(df, layer3, "TOP")
lyr3 = arcpy.mapping.ListLayers(mxd, "", df)[0]
lyr3.visible = False
lyr3.name = "MW_Decision"
mxd.saveACopy(r"C:\Users\Documents\NGN\DO Maps\South.mxd")
arcpy.RefreshActiveView()
arcpy.RefreshTOC()
del mxd,df
print "save South mxd"