MOBILE4 is a computer program that calculates emission factors for hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) from highway motor vehicles. MOBILE4 calculates emission factors for eight individual vehicle types, in two regions of the country (low- and high-altitude). The emission factors depend on various conditions such as ambient temperature, fuel volatility, speed, and mileage accrual rates. MOBILE4 will estimate emission factors for any calendar year between 1960 and 2020 inclusive. The 20 most recent model years are considered in operation in each calendar year. MOBILE4 supersedes MOBILE3. Relative to MOBILE3, MOBILE4 incorporates several new options, calculating methodologies, emission control regulations, and internal program designs. Software Description: The software is written in the FORTRAN language for use on an IBM PC or compatible machines using MS-DOS operating system. It requires a minimum of 920K of memory.