SFIT4 is a radiative transfer and atmospheric constituent profile retrieval algorithm for use with spectra recorded in solar absorption and emission across the infrared spectrum. The predecessors of SFIT4 (SFIT & SFIT2) were co-developed over time at NASA Langley, NIWA Lauder NZ and the Univ. of Denver (Chang & Shaw, 1977, Niple, 1980, Goldman et al., 1980, Pougatchev & Rinsland, 1995). This version of SFIT4 v0.9.4 implements a line-by-line code with 1st order line-mixing and the generalized lineshape profile algorithm pCqSDHC (Ngo, 2013) and has the possibility of explicit Total Internal Partition Function (TIPS) calculations (Fischer, 2003). It implements upwards looking geometry using Fourier Transform interferometer spectra. For the retrieval of atmospheric chemical profiles the Optimal Estimation (Rodgers 2000) is implemented. SFIT4 allows the use of a user defined regularization matrix. The raytracing algorithm (FASCATM) is integrated in the code (Smith, et. al., 1978, Gallery, et. al., 1983, Shephard, et. al., 2009). SFIT4 is one of two standard retrieval codes employed by the NDACC IRWG (NDACC Infrared Working Group) and consequently operationally employed for satellite and model validation, trend evaluation and broad range of atmospheric research purposes. The SFIT4 retrieval algorithm is designed to work with spectroscopic data from HITRAN (Rothman 2005, Rothman 2013) and ATM (Toon 2013). The algorithm is driven with an a-priori state of the atmosphere (pressure, temperature and volume mixing ratio profiles for the atmospheric constituents). A-priori input profiles for selected sites are extracted from WACCM v6 (Gettelman 2019). The source code can be obtained from the links below or from https://wiki.ucar.edu/display/sfit4/SFIT+Core+Code.