Hi Try This
cmd = new SqlCommand("select Eid,Ename from temp order by Eid ";,con);
cmd.CommandType = CommandType.Text;
dr = cmd.ExecuteReader();
if (dr.HasRows == true)
{
while (dr.Read())
name.Add(dr["Eid"].ToString());
name.Add(dr["Ename"].ToString());
con.Close();
textBox4.AutoCompleteMode = AutoCompleteMode.Suggest;
textBox4.AutoCompleteSource = AutoCompleteSource.CustomSource;
textBox4.AutoCompleteCustomSource = name;
}