How to use dataset column that is not a bound field in gridview for rowbound condition

<style>
       .hide {
           display:none
       }
   </style>

<asp:GridView  ID="GridView1" AutoGenerateColumns="false" runat="server" OnRowDataBound="GridView1_RowDataBound">
               <Columns>

                   <asp:BoundField DataField="a1" HeaderText="a1" />
                    <asp:BoundField DataField="a2"  HeaderText="a2" />
                    <asp:BoundField DataField="a3"  HeaderText="a3" />
                   <asp:BoundField DataField="a4" ItemStyle-CssClass="hide" HeaderStyle-CssClass="hide" />

               </Columns>
           </asp:GridView>

protected void Page_Load(object sender, EventArgs e)
       {
           DataTable dt = new DataTable();
           dt.Columns.Add("a1");
           dt.Columns.Add("a2");
           dt.Columns.Add("a3");
           dt.Columns.Add("a4");
           dt.Rows.Add(1, 2, 3, 4);
           dt.Rows.Add(11, 22, 33, 44);
           GridView1.DataSource = dt;
           GridView1.DataBind();
       }

       protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
       {
           if (e.Row.RowType == DataControlRowType.DataRow)
           {
               string a1 = e.Row.Cells[0].Text;
               string a2 = e.Row.Cells[1].Text;
               string a3 = e.Row.Cells[2].Text;
               string a4 = e.Row.Cells[3].Text;

           }
       }