How to use Rowspan in Gridview for 1st Column only

Use RowDataBound event instead:

void GridView31_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow )
    {
        if (e.Row.RowIndex % 4 == 0)
        {
            e.Row.Cells[0].Attributes.Add("rowspan", "4");
        }
        else
        {
            e.Row.Cells[0].Visible = false;
        }
    }
}