re: "When the walls and floors are all built, those four 2x4s are removed. "
Except that the 2 x 4's are in the way of the walls so they have to be removed first.
One way to accomplish this is to remove the bottom half of the 2 x 4's (or the top half, your choice) - all four at once - and then quickly (very quickly! ) build a half wall all around the house to support the top half of the 2 x 4's, unless you removed the top halves of the 2 x
4's, in which case you set a half wall on top of the bottom halves of the 2 x 4's and set the roof on the half walls.Once the first half walls are in place, you remove the other half of the 2 x 4's and quickly (again, very quickly!) build the top (or bottom) half of the walls.
QED