Four tree support examples
Building a treehouse that can accommodate movement in four trees is quite difficult. The hardest way to go about things is to use a bracket system such as that shown in the diagram on the right.
A cable system working in combination with a couple of metal brackets will take the best from both systems. Remember that brackets are good for the stability of the treehouse, and cables are good for awkward trees and points that move around a lot. Don't let things get too complicated or you will increase the amount of maintenance needed further down the line, and once a treehouse is in position it can be a big operation to repair supports that are underneath. You may also decide to avoid one or more of the trees and opt for a simpler support method with fewer trees, or use posts to provide some of the support.
Floor supports using brackets and floating platform
In this bracket-based system the main floor is a rigid frame of joists, reinforced with a diagonal brace. The beam spanning the gap between the right hand trees is underneath the two 'arms' that extend to the right of the floor. The sliding surfaces are faced with metal to prevent wear. This allows the framework to take up movement in all directions. The two main beams (left and right) have one end fixed to a tree, and the other end in a bracket, allowing the trees to move independently. Altogether, this combination of fixed and flexible joints allows the platform to stay a fixed shape, but still move around with the trees.
Cable system with one fixed joint
When cables are used all the main support beams can be in the same plane. The platform is built as one solid piece which fits closely around the trees. One corner is fixed to the least-moving tree (usually the largest) and the other three corners are suspended from points in the remaining three trees. This system is simpler to design and build than the bracket system above, and is more easily adjusted when the treehouse is built on top. However, cables have no resistance to sideways motion, so even people walking inside can cause the treehouse to noticeably move around on the cables.