ElectroServer 4 is a free server that allows up to 25 connected users. It allows client applications like Flash to connect via socket and log in. Developers can extend its functionality through extensions, which are ActionScript or Java files that add features. It has unprecedented scalability, supporting hundreds of thousands of players by distributing load across gateway servers. It also enables features like audio/video streaming and recording for live events. Licensing is tiered based on concurrent users, making it suitable for applications of all scales.