RSP4J is an API for RDF stream processing that addresses gaps in existing solutions. It provides an extensible architecture, declarative access to streams and queries, and programming abstractions to enable fast prototyping, benchmarking, and dissemination of RSP techniques. The API generalizes common operators, implements execution semantics of engines like YASPER and CSPARQL 2.0, and allows controlling query execution. RSP4J aims to foster practical RSP research by simplifying the implementation of new solutions, engines, and optimizations.