Generar una matriz de números y mostrar la diagonal proncipal

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim areglo(4, 4) As Integer
        Dim i, j As Integer
        Dim contador As Integer
        Dim promp As String = "Arreglo bidimencional"
        Dim mensaje As String = "Leer los elementos del arreglo"
        Dim text As TextBox() = New TextBox() {TextBox1, TextBox2, TextBox3, TextBox4, TextBox5, TextBox6, TextBox7, TextBox8, TextBox9, TextBox10, TextBox11, TextBox12, TextBox13, TextBox14, TextBox15, TextBox16}
        Dim suma As Integer = 0
        Dim prom As Decimal = 0
        Dim mayor As Integer = 0
        contador = 0
        For i = 0 To 3
            For j = 0 To 3
                areglo(j, i) = InputBox(mensaje, promp)
                text(contador).Text = areglo(j, i)
                contador = contador + 1
                If mayor < areglo(j, i) Then
                    mayor = areglo(j, i)

                End If

            Next j

        Next i
        suma = areglo(0, 0) + areglo(1, 1) + areglo(2, 2) + areglo(3, 3)
        prom = suma / 4
        MsgBox("Los numeros capturados de la diagonal son:  " & areglo(0, 0) & " - " & areglo(1, 1) & " - " & areglo(2, 2) & " - " & areglo(3, 3))

    End Sub
End Class