I can not see your layers, but perhaps you are miscounting the layers? The group counts as a layer item in the table of contents, SO if you had your first layer and then a group layer, and within the group layer you 3 layers, Roads, Streams, Houses, a pointer to the Houses layer would be [4]. You can verify the layer pointer with a simple print or arcpy.AddMessage statment like print arcpy.mapping.ListLayers(mxd)[7], and see if it returns what you expect.
Regards,
Jim