From my past experience in making wedding cakes, each tier would be two layers of cake.
Wedding cake pans make cake layers that are 2 inches high, so two layer of these wedding cake layers would be 4 inches high (filling, frosting, and decorations make it taller).
The first cake would be 4 total layers, the second cake would be 6 total layers.
You can also cut the layers in half if you want to make special fillings for each layer. So that would make the first cake 8 layers and the second cake 12 layers.