Исправлено сейчас!
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
For Each f As Form In Application.OpenForms
If TypeOf f Is Form4 Then
f.Activate()
Exit Sub
End If
Next
Dim MDIForm As New Form4
MDIForm.MdiParent = Form2
MDIForm.Show()
End Sub
Я определял родителя MDI в неправильной форме!