In addition to engine efficiency, the power output is dependent on the amount and energy content of the fuel that is burned. Therefore, to know or be able to estimate the power output of a 4-cylinder engine, you will have to provide a little more information. The Carnot cycle efficiency for any engine represents the maximum efficiency that can be approached theoretically. This efficiency is equal to one minus the ratio of the absolute temperature of the burning fuel and the absolute temperature of the surroundings. Some cars, like small K-cars, have 1.9 or 2.0 liter engines which have about 100 horsepower or so.
Fuel injection can also play a large role in power.
Overall though, horsepower is usually defined by or is proportional to the number of RPM's the engine can sustain. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.