Ошибка в программе, меняющей местами максимальный и минимальный элементы матриц A размера 4x4

Sub mesta()
Dim i As Integer
Dim j As Integer
Dim i_min As Integer
Dim j_min As Integer
Dim i_max As Integer
Dim j_max As Integer
Dim a(1 To 4, 1 To 4) As Integer
For i = 1 To 4
For j = 1 To 4
a(i, j) = InputBox("Ââåäèòå a(" & i & "," & j & ")")
Range("A2:D5").Value = a
MsgBox ("áûë ââåäåí " & a(i, j))
Next j
Next i
For i = 1 To 4
For j = 1 To 4
If a(i, j) > a(i_max, j_max) Then   Почему выдает ошибку №9(subscript out of range)?
i_max = i
j_max = j
End If
If a(i, j) < a(i_min, j_min) Then    Почему выдает ошибку №9(subscript out of range)?
i_min = i
j_min = j
End If
Next j
Next i
Dim f As Integer
a(i_max, j_max) = a(i_min, j_min)
a(i_min, j_min) = f
Range("F2:I5").Value = f
End Sub

Sub mesta()
 Dim a(1 To 4, 1 To 4) As Integer
 Dim i As Integer
 Dim j As Integer
 Dim MinZn As Integer
 Dim MaxZn As Integer
 Dim msgStr As Integer
 Dim y As String
 For i = 1 To 4
 For j = 1 To 4
 a(i, j) = InputBox("Ââåäèòå a(" & i & "," & j & ")")
 Range("A2:D5").Value = a
 Next j
 Next i
 For i = 1 To 4
 For j = 1 To 4
 If a(i, j) < MinZn Then MinZn = a(i, j)
 If a(i, j) > MaxZn Then MaxZn = a(i, j)
 Next j
 msgStr = msgStr & vbCrLf
 Next i
 For i = 1 To 4
 For j = 1 To 4
 If a(i, j) = MinZn Then
 a(i, j) = MaxZn
 End If
 If a(i, j) = MaxZn Then
 a(i, j) = MinZn
 End If
 y = y & a(i, j) & " " & CStr(v)
 Next j
 y = y + vbCr
 Next i
 PushButton = MsgBox(y, vbOKOnly, "ìàòðèöà ñ çàìåíîé min íà max A")
 End Sub