The authors present a second-generation broadband 4×4 Mueller-matrix (MM) ellipsometer for ultrasensitive infrared-spectroscopic (8000−800cm−1) studies of complex nanometer-thin films. In a modular design, the instrument employs retractable achromatic retarders and various sets of tandem polarizers. Using high-transmittance free-standing wire-grid polarizers, the device reaches an unparalleled precision of up to 5⋅10−5 in the important fingerprint region, even for block-offdiagonal MM elements. Broadband and signal-to-noise optimized access to the full 4×4 MM provides in-depth information on the sample’s polarimetric properties and opens the door for detailed explorations of depolarizing and anisotropic materials. The authors discuss examples of highly depolarizing nonuniform polyimide membranes, uniaxial-to-biaxial anisotropy changes in ultrathin polymer films, and azimuthal off-axis effects in 2D-structured silica arrays. Diverse optical modeling approaches based upon anisotropic layer stacks and rigorous coupled-wave analysis are used to quantify the optical, structural, and chemical properties of the sample.