*Call for Participation:* Machine Learning from User Interactions (MLUI) workshop at IEEE VIS 2021 The Machine Learning from User Interactions (MLUI) workshop is back for a 4th year. In this workshop, we seek to bring together researchers to share their knowledge and build collaborations at the intersection of the Machine Learning and Visualization fields, with a focus on learning from user interaction. Rather than focusing on what visualization can do to support machine learning (as in current Explainable AI research), this workshop seeks contributions on *how machine learning can support visualization*. Such support incorporates human-centric sensemaking processes, user-driven analytical systems, and gaining insight from data. Our intention in this workshop is to generate open discussion about how we currently learn from user interaction, how to build intelligent visualization systems, and how to proceed with future research in this area. We hope to foster discussion regarding systems, interaction models, and interaction techniques. MLUI 2021 largely plans to continue the momentum in this research area that the previous workshops have initiated. This year, we plan to accept both short and long papers jointly in the same submission block. Full papers have a length of 5-10 pages (not including references), while short papers are 2-4 pages (plus references). Short papers are intended to capture either (1) limited aspects of a larger work that fit our call or (2) late-breaking work not yet mature enough for a full paper submission. The option of submitting a short paper replaces the posters track that we previously offered at MLUI. Submissions will be reviewed by a committee of reviewers that we will organize. This committee will include the workshop committee members. The size of the committee will be determined by the number of submissions, such that each submission is reviewed by at least 2 committee members. Both full and short paper metadata (author information, title, university, etc.) as well as the submissions themselves will be posted to the workshop website in advance of the event. Workshop papers will be archived on IEEE Xplore following the conference. *Website:* https://learningfromusersworkshop.github.io/ *Submission deadline:* July 30, 2021 *Author notification:* August 15, 2021 *Workshop dates:* TBD, held with IEEE VIS (October 24-29, 2021)