MadGraph/MadEvent v4 is a new web-based event generator for hadron collider physics. It allows users to generate events at the parton, hadron, and detector levels directly from a web interface for Standard Model and beyond-the-Standard-Model processes. Key features include a new framework for user-defined models, standalone running modes, simultaneous generation of signal and background processes, two data analysis platforms, and inclusive multi-jet samples via parton showers. The paper presents applications to Higgs searches, CP properties, spin of new resonances, single-top and Higgs production in 2HDM, SUSY pair production, and W+jets samples. The package includes tools for analysis, decay of unstable particles, and hadronization. It supports models like SM, MSSM, 2HDM, and HEFT, with a user model framework. The implementation includes Standard Model interactions, Higgs Effective Theory, and new physics models. The package allows for flexible event generation, with options for web-based or local use, and provides tools for data analysis and simulation. The paper discusses the structure, models, and applications of the package, highlighting its capabilities in simulating collider physics processes.MadGraph/MadEvent v4 is a new web-based event generator for hadron collider physics. It allows users to generate events at the parton, hadron, and detector levels directly from a web interface for Standard Model and beyond-the-Standard-Model processes. Key features include a new framework for user-defined models, standalone running modes, simultaneous generation of signal and background processes, two data analysis platforms, and inclusive multi-jet samples via parton showers. The paper presents applications to Higgs searches, CP properties, spin of new resonances, single-top and Higgs production in 2HDM, SUSY pair production, and W+jets samples. The package includes tools for analysis, decay of unstable particles, and hadronization. It supports models like SM, MSSM, 2HDM, and HEFT, with a user model framework. The implementation includes Standard Model interactions, Higgs Effective Theory, and new physics models. The package allows for flexible event generation, with options for web-based or local use, and provides tools for data analysis and simulation. The paper discusses the structure, models, and applications of the package, highlighting its capabilities in simulating collider physics processes.