The question asks for the approximate wavelength of violet light in air. Violet light is a color at the shorter end of the visible light spectrum, meaning it has a relatively shorter wavelength.

In this question, the correct answer is option 4, 0.4 µm. The symbol `µm` represents micrometers, which is equal to 10^-6 meters. Thus, 0.4 µm is equivalent to 0.4 x 10^-6 meters.

It`s important to note that the approximate wavelength of violet light may vary slightly depending on different sources and definitions. However, in general, violet light has wavelengths ranging from approximately 400 to 450 nanometers (nm) or 0.4 to 0.45 micrometers (µm).

Therefore, option 4, 0.4 µm, is the closest approximation to the wavelength of violet light in air.