With the current and expected growth of air traffic, the need for increased support for air traffic controllers from automation continues to grow. One method of improving air traffic management applies 4D trajectory management to control traffic in real-time. An interface applying this method only allowed for manipulation of trajectory in three dimensions, not supporting altitude. In order for the interface to allow for complete 4D manipulation of the trajectory, the altitude of the aircraft trajectories, as well as the vertical travel space, need to be visualized and manipulation of the altitude needs to be supported. Using the principles of ecological interface design, the existing interface was expanded by adding a Vertical Situation Diagram (VSD). This interface was validated in an experiment where the control strategies were observed as well as metrics on safety, efficiency and user workload. A comparison with an experiment without VSD hints at a decrease in workload for participants while solving conflicts at least as efficiently, if not more efficiently. The VSD is a vital part of the interface, not only for completion's sake but also to achieve improved performance, especially when moving towards more realistic scenarios.