C#

if ((MenuItem)sender) == mnuItem1) ||
(MenuItem)sender) == mnuItem2) ||
(MenuItem)sender) == mnuItem3) ||
(MenuItem)sender) == mnuItem4))

class RichTextBox : System.Windows.Forms.RichTextBox
{
  void ScrollToEnd()
  {
    Point pt2 = GetPositionFromCharIndex(0);
    Point pt = GetPositionFromCharIndex(TextLength);
    const int msg = 0x400 + 222;
    Point point = new Point(0, Font.Height + pt.Y - pt2.Y - Height);
    SendMessage(Handle, msg, 0, ref point);
  }

  [DllImport("user32.dll")]
  extern static int SendMessage(IntPtr hwnd, int id, int res, ref Point pos);

  protected override void OnSizeChanged(EventArgs e)
  {
    ScrollToEnd();
    base.OnSizeChanged (e);
  }

  //this scrolls the text to the last line when ever text is changed.
  protected override void OnTextChanged(EventArgs e)
  {
    ScrollToEnd();
    base.OnTextChanged (e);
  }
}

richTextBox1.Select(richTextBox1.Text.Length, 0);
richTextBox1.ScrollToCaret();

richTextBox1.Focus();

Remarks<br />
This method enables you to scroll the contents of the control until the caret is within the visibile region of the control. If the caret is positioned below the visible region of the control, the ScrollToCaret method will scroll the contents of the control until the caret is visible at the bottom of the control. If the caret is positioned above the visible region of the control, this method scrolls the contents of the control until the caret is visible at the top of the control. You can use this method in a multiline text box to ensure that the current text entry point is within the visible region of the control.<br />
<br />
Note   This method has no effect if the control does not have focus or if the caret is already positioned in the visible region of the control.<br />

TextBox tb = new TextBox();
tb.MaxLength = 0; // or any value > 32K
tb.Text = new string('0', 32767);
tb.AppendText("12345");
System.Diagnostics.Trace.Assert(tb.Text.Length == 32767+5);