iSilo 4.x: The iSilo 4.x document format builds on the iSilo 3.x document format with extended capabilities supported by and to be supported by iSilo 4.x. Documents in the iSilo 4.x document format can also be opened and read by iSilo 3.x. iSilo 3.x: The iSilo 3.x document format was designed from the ground up for efficiency with the needs and limitations of handheld devices in mind. Its new architecture provides a flexible and powerful foundation for growth in features and functionality into the foreseeable future. iSilo 2.x: The iSilo 2.x document format was at the forefront of providing second generation document display for Palm OS handhelds and still does remain a viable platform for doing so.