A 16 valve engine is not necessarily a 4 cylinder. The number of valves an engine has is not directly related to the number of cylinders it has.
A lot of 4-cylinder engines have 16 valves. However, there are also 4-cylinder engines with fewer valves, such as 8 or 12 valves. Similarly, there are engines with more than 4 cylinders that have 16 valves, such as some V6 or V8 engines.
The valve count is determined by the engine designer's goal to optimize the engine's performance. This is influenced by several factors such as combustion chamber shape, piston design, and fuel delivery systems. Ultimately, the design and configuration of an engine depend on its intended application and the goal of the manufacturer.