I'm a little confused I want to start off on the right foot and get every tier the exact same. If the tier is 4" then is that the finished (filled and iced) or just the sponge.
I have just baked two vanilla sponges, If I trim off the top dome should the total be 4" or 3.5". There will be 4 layers then.
I try to get mine to 4 inches after filling and icing, but before fondant. That's another 1/4 inch, I guess.
One of the reasons I need to get it correct is that I may use cigarellos if my plan for chocolate strips (like collars but tempered chocolate taken off a frozen marble slab placed on and built up). If my plan fails I can always fall back on the cigarello idea so these are 12" in height.
I've worked out then then each sponge should be trimmed to 4.25cm - this allows 1.5 for granache - If that's correct?
It is really up to your preference. There is no mandatory finished height. Usually they are 3 1/2 or so or higher.
I don't use quite that much ganache, but you certainly could. The finished height is really up to you, as long as all the tiers are the same. Unless they're not supposed to be.