This open access four-volume set constitutes the refereed proceedings of the Fourth World Conference on Explainable Artificial Intelligence, xAI 2026, held in Fortaleza, Brazil, during July 1–3, 2026.

The 95 revised full papers presented in these proceedings were carefully reviewed and selected from 235 submissions. The papers are organized in the following topical sections:

Volume I: Enhancing Models with XAI: Debugging, Modification, Evaluation; Human-Centred Explanation Systems & Explanation User Interfaces; Rule-Based Explainable AI; Uncertainty-Aware Explainable AI; xAI in Healthcare: Balancing Accuracy, Trust, and Ethics.

Volume II: XAI for Time Series; Trust Through Transparency: Next-Generation Explainable AI in Medicine; Explainable AI for Sequential Decision Making;  Mechanistic Interpretability in Explainable AI; XAI and the Physical Sciences.

Volume III: Psychometric Applications for XAI and attribution techniques; Neural-Symbolic Methods for Interpretable and explainable AI; Argumentation, abduction, abstraction and agreement for XAI; Shapley-driven explainable AI; Explainable AI in legal and regulatory contexts.

Volume IV: Counterfactual-based explainable AI; Explainability with and for Large Language Models; Notions and metrics for explainable AI; Contrastive-based explainable AI; Novel approaches and methods for XAI.