Consider the following statements regarding the 'strength of cement':

1. Strength test on cement is made on cubes of the cement-sand mix.

2. Water to be used for the paste is 0.2P, where P is the water needed for normal consistency.

3. The normal consistency is determined on Le Chatelier's apparatus.

4. Cubes are cast in three layers in leak-proof moulds further compacted in each layer by vibration on a machine.

Which of the above statements are correct?