The spectral series corresponds to groups of wavelengths that are produced when electrons in an atom make transitions between energy levels. Each series is named after the scientist who discovered it and is characterized by the energy level to which the electrons transition.

The transitions to the level $n_1 = 4$ define the Brackett series. Electrons that fall to the fourth energy level (from higher levels with $n_2 = 5, 6, \ldots$) emit or absorb radiation in the infrared region of the electromagnetic spectrum.

Here's a quick summary of the different spectral series:

• Pfund series: Transitions to $n_1 = 5$
• Brackett series: Transitions to $n_1 = 4$
• Lyman series: Transitions to $n_1 = 1$
• Balmer series: Transitions to $n_1 = 2$

Therefore, the correct answer is:

Option B: Brackett series