What is the main difference between a spark-ignition petrol engine and a compression-ignition diesel engine?

1. Diesels use a spark plug to ignite fuel; petrol engines never compress air
2. Petrol SI engines ignite the mixture with a spark; diesel CI engines ignite fuel in hot compressed air without a spark plug spark for ignition
3. Diesels cannot have four strokes
4. Petrol engines do not have pistons