To determine which of the listed properties are physical tests employed for the manufacture of paper, we need to understand what each property entails and how it relates to the production and quality of paper.

1. Mechanical and strength properties: These are crucial for understanding how paper will perform under various conditions. These properties include tensile strength, tear resistance, and burst strength, which are essential to ensure the durability and usability of the paper. Therefore, these are indeed physical tests used in paper manufacturing.
2. Surface properties: The surface properties, such as smoothness and texture, are critical to the printing quality and writing performance on the paper. Tests for surface properties help in assessing how paper will interact with inks and writing materials. Thus, these are essential physical tests in paper manufacturing.
3. Optical properties: Optical properties like brightness, whiteness, and opacity are important for visual appearance and printability. Testing these properties ensures that the paper meets the desired quality standards in terms of look and readability. Therefore, optical properties are also part of the physical tests.
4. Permeability to fluids: This property indicates how well the paper resists penetration by liquids, which is crucial for uses like packaging and other applications where moisture or liquid contact is possible. Testing the permeability is an essential physical check in paper production.

All the listed properties are relevant and important for testing paper during and after manufacturing. Hence, the correct answer is I, II, III and IV.