IslandViewer 4 is a webserver for predicting and visualizing genomic islands (GIs) in bacterial and archaeal genomes. It provides enhanced features for larger-scale microbial genomics analysis, including improved GI predictions and a flexible visualization interface. A user management web interface and HTTP API for batch analyses are now available, allowing secure submission of larger numbers of genomes and retrieval of results. The integrated GI predictions from multiple methods have been improved and expanded by incorporating the precise Islander method for pre-computed genomes and an updated IslandPath-DIMOB for both pre-computed and user-supplied genomes. Pre-computed predictions including virulence factors and antimicrobial resistance are now available for 6193 complete bacterial and archaeal strains in RefSeq. IslandViewer 4 includes predictions from the Islander database, a fourth highly precise method that specifically predicts GIs integrated into tRNA and tmRNA genes. It also integrates an improved version of IslandPath-DIMOB with updated Pfam profiles and more stringent significance cutoffs to avoid false positives. The new version provides a 11.7% increase in recall and a 5.3% increase in precision compared to the previous version. IslandViewer 4 has a flexible interactive interface for visualizing GIs and their gene content using GenomeD3Plot. It also includes a new approach to speed up the loading of GI prediction results and a streamlined layout for the circular view to produce publication-grade figures. The web and programmatic user interface now allow for larger-scale analysis of genomes, with an HTTP API for batch submissions and retrieval of results. The use of MASH for genomic distance calculations has been integrated to enable faster time-to-results for GI predictions. IslandViewer 4 also includes a new database backend using MariaDB, which provides performance enhancements and improves scalability. It is the only currently active web service enabling GI analysis for draft and complete genomes. The improvements in IslandViewer 4 have made it possible to integrate it into the pipeline of the Integrated Rapid Infectious Disease Analysis platform used by public health agencies for pathogen outbreak analysis. Overall, the improvements proposed by IslandViewer 4 will enhance the study of GIs and their role in the evolution of a wide range of bacterial and archaeal species of interest and investigations into the acquisition of antimicrobial resistance and other medically important adaptations.IslandViewer 4 is a webserver for predicting and visualizing genomic islands (GIs) in bacterial and archaeal genomes. It provides enhanced features for larger-scale microbial genomics analysis, including improved GI predictions and a flexible visualization interface. A user management web interface and HTTP API for batch analyses are now available, allowing secure submission of larger numbers of genomes and retrieval of results. The integrated GI predictions from multiple methods have been improved and expanded by incorporating the precise Islander method for pre-computed genomes and an updated IslandPath-DIMOB for both pre-computed and user-supplied genomes. Pre-computed predictions including virulence factors and antimicrobial resistance are now available for 6193 complete bacterial and archaeal strains in RefSeq. IslandViewer 4 includes predictions from the Islander database, a fourth highly precise method that specifically predicts GIs integrated into tRNA and tmRNA genes. It also integrates an improved version of IslandPath-DIMOB with updated Pfam profiles and more stringent significance cutoffs to avoid false positives. The new version provides a 11.7% increase in recall and a 5.3% increase in precision compared to the previous version. IslandViewer 4 has a flexible interactive interface for visualizing GIs and their gene content using GenomeD3Plot. It also includes a new approach to speed up the loading of GI prediction results and a streamlined layout for the circular view to produce publication-grade figures. The web and programmatic user interface now allow for larger-scale analysis of genomes, with an HTTP API for batch submissions and retrieval of results. The use of MASH for genomic distance calculations has been integrated to enable faster time-to-results for GI predictions. IslandViewer 4 also includes a new database backend using MariaDB, which provides performance enhancements and improves scalability. It is the only currently active web service enabling GI analysis for draft and complete genomes. The improvements in IslandViewer 4 have made it possible to integrate it into the pipeline of the Integrated Rapid Infectious Disease Analysis platform used by public health agencies for pathogen outbreak analysis. Overall, the improvements proposed by IslandViewer 4 will enhance the study of GIs and their role in the evolution of a wide range of bacterial and archaeal species of interest and investigations into the acquisition of antimicrobial resistance and other medically important adaptations.