Sub test2()
Dim n As Long, t, i As Long, j As Long, k As Long
Application.ScreenUpdating = False
n = Range("A" & Rows.CountLarge).End(xlUp).Row
t = Range("a4:c" & n)
ReDim r(1 To 12 * UBound(t), 1 To UBound(t, 2) + 1): n = 0
For i = 1 To UBound(t)
For k = 1 To 12
n = n + 1
For j = 1 To UBound(t, 2): r(n, j) = t(i, j): Next j
r(n, UBound(r, 2)) = k
Next k
Next i
Range("a4").Resize(UBound(r), UBound(r, 2)) = r
End Sub