Consider the following statements about the development and features of the periodic table:

1. Mendeleev's Periodic Law was based on the atomic number of elements.
2. Moseley's work established the concept of atomic number as the basis for the modern periodic table.
3. The modern periodic table includes synthetic elements that Mendeleev could not predict.
4. Mendeleev left gaps in his table, predicting the properties of undiscovered elements like gallium and germanium.

Which of the above statements is/are correct?