The incorrect statement among the following is:

1.

A molecule of a compound contains atoms from various elements.

2.

A compound cannot be broken down into its constituent elements using physical separation techniques.

3. A compound retains the physical properties of its constituent elements.
4. The ratio of atoms of different elements in a compound is fixed.
Hint: Compound contains two or more than two elements combined together in a fixed proportion by mass.

Explanation:
A compound is a pure substance containing two or more than two elements combined together in a fixed proportion by mass and which can be decomposed into its constituent elements by suitable chemical methods.
 
Further, the properties of a compound are quite different from the properties of constituent elements.
 
e.g., water is a compound containing hydrogen and oxygen combined together in a fixed proportion.

But the properties of water are completely different from its constituents, hydrogen and oxygen. Hence, option 3 is the correct answer.