Preface: 4th Workshop on Natural Language Processing for Requirements Engineering (NLP4RE 2021)

The Natural Language Processing for Requirements Engineering Workshop (NLP4RE) was established in 2018 as a venue to foster communication between researchers and practitioners interested in the field. The 2021 edition was held virtually in Essen, due to the COVID-19 pandemic, and saw the presentation of 10 papers covering different aspects of NLP4RE, including information extraction (e.g., rationale, causality), requirements classification and chat-bots. The workshop saw a lively participation, with over 25 participants during the keynote and about 20 participants during the paper presentation sessions.