There's no single answer to what size engine uses a 4-cylinder, 8-spark plug configuration. This setup is unusual. Most 4-cylinder engines have 4 spark plugs (one per cylinder), or sometimes 2 per cylinder (for instance, in some high-performance engines), for a total of 8 only if it's a V8 (which has 8 cylinders).
An 8-spark plug configuration in a 4-cylinder engine suggests a very specialized, likely high-performance, or even experimental engine design with dual ignition systems in each cylinder. There's no common engine size associated with this type of setup.
