I do 3 layers personally. I use one of those cake levelling things where you can adjust the size. I have it set to about 1 1/4 inch. Then I know they will be nice even layers. Once stacked they will be 3 3/4 inch but once they have layers of buttercream in between and a layer of fondant over the top that brings me up to 4 inch. I stick with 4 inch for celebration cakes but often bump it up to 4 1/2 inch (occasionally 5inch if design calls for it) for weddings. Hope that helps. x
CakesWorth
