Dataflow visualization systems such as AVS, IRIS Explorer, and IBM Data Explorer have been widely used. But the inherent problems such as excessive memory usage and low run- time efficiency have imposed limitations for their use with large, time-dependent data sets. For interactive visualization, efficiency is more of a necessity than luxury. 4d2 is designed as a high-performance interactive visualization tool for time-dependent 3D CFD data on rectangular grids containing multiple scalar and vector fields, as well as for time-dependent 3D particle data. The visualization functions include contour plot, vector plot, slicing, iso- surface extraction, streamline integration, particle displaying, and volume rendering, as well as graphical user interface and animation control for time-dependent data sets. Several features distinguish 4d2 from other visualization tools. First, it provides a rich set of visualization functions and allow mixing of volume rendering with iso-surfaces, streamlines, particles, and so on. Secondly, many techniques are used to achieve extremely memory-efficient and time- efficient run-time performance: no data duplication is needed, hardware alpha-buffering is used to do volume-rendering, immediate graphics mode is used, and parallel algorithms are used when multiprocessors are available. This GL-based software runs on SGI workstations with both 8-bit and 24-bit graphics hardware, and is available in public domain. 4d2 has been successfully used in atmospheric science and astrophysics at the National Center for Supercomputing Applications and a number of other research labs.