The Genome Analysis Toolkit (GATK4) is a production-grade bioinformatics framework developed by the Broad Institute for variant discovery and genotyping in high-throughput sequencing data. It provides a comprehensive suite of command-line tools for germline and somatic variant calling (HaplotypeCaller, Mutect2), copy number variation analysis (GermlineCNVCaller), structural variant detection (FindBreakpointEvidenceSpark), functional annotation (Funcotator), base quality score recalibration (BQSR), and read processing. The architecture is built around a plugin-style engine with walker base classes (ReadWalker, VariantWalker, LocusWalker) that define traversal patterns over genomic data, with extensive use of Apache Spark for distributed execution on clusters and cloud platforms (GCS, HDFS). Key entry points include the `gatk` wrapper script and individual tool classes registered via the Barclay argument parser, with workflows orchestrated through WDL/Cromwell pipelines. External dependencies include htsjdk for BAM/VCF I/O, BWA for alignment, PyMC3 for Bayesian CNV modeling, and PyTorch for deep learning-based variant scoring. The codebase is heavily tested with unit and integration tests, and supports both local and cloud-based execution.