Why only a double layer cake? Is the cake so moist and airy I won't be able to split the separate layers in order to have four layers? Can anyone...

I find an easy solution to more layers is often just to divide the batter into more pans. I frequently bake 3 or 4 layers from a 2-layer recipe. Just shorten the baking time and watch it carefully. They will dry out quickly if overbaked.

Piece of Layer Cake provides a great answer with good information.

I believe a 2-layer cake is mentioned in the recipe because that is the most common, classic "American-style" cake for a home baker. More experienced types will go for more layers, but this is perhaps the standard.

If a cake can be layered, it can be split. In an effort to reach a wide audience, many recipe authors try to provide decent recipes while keeping them as simple as possible. Nothing is more simple than baking two cakes and sandwiching them together. I would venture to say that any butter cake can be easily split. THIS cake in particular has a instruction that has you soak the cake in syrup after piercing it with a skewer. If you plan on splitting this cake, I would skip that step and maybe just brush the syrup on the split (and completely cooled) layers for a bit of extra flavor and moistness. But be careful not to go crazy or let the cake layer sit for too long or you might have a mess.

I would bake the two cakes a day ahead, wrap them well and chill them overnight. That will make them easy to split. Split, frost and decorate the cake chilled but serve at room temperature. If you do make 4 layers out of this, it will probably need a bit more buttercream than a 2 layer cake. 4 - 5 cups, I would say, but I always make extra for decorating....and extra can be frozen.

Some recipes use buttermilk, but I've also seen recipes use sour cream, yogurt, crème fraîche, etc. The basic concept is that the dairy enriches the batter while the acidity leads to some lift and a more tender crumb. I have a thousand different cake recipes if you need something specific, and I've never made this particular cake, but I'd give it a go. Let us know!