You don't need 4m to fit a Savannah, but it makes a more useful shed of it. Mine has two tractors, a round hay baler and a spray rig in it as well, so make it a useful height. Mine is 4.5m eave height and 3.9m through the side roller door and about 4.3m at the sliding doors on the gable. One or more whirly birds have blown off it three times now (buy the dear ones!), but the shed hasn't looked like blowing away. It is positioned with a solid sheeted wall towards the prevailing south westerly of the nasty storms, I think that is important.