Public Shared frm1 As Form1
 
    Private Sub MenuItem4_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MenuItem4.Click
        Dim f2 As New Form2()
        frm1 = Me
        Me.MenuItem4.Enabled = False
        f2.MdiParent = Me
        f2.Show()
    End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        Form1.frm1.MenuItem4.Enabled = True
        Me.Close()
    End Sub