ST4R-Splat: Spatio-Temporal Referring Segmentation in 4D Gaussian Splatting

Understanding objects in dynamic 4D environments via natural language is crucial yet underexplored. While existing methods focus on static 3D referring segmentation or openvocabulary 4D querying, they struggle to ground complex spatio-temporal referring expressions in explicit 4D reconstructions. We introduce Spatio-Temporal Referring Segmentation in 4D Gaussian Splatting (STRS-4DGS), a novel task aiming to jointly identify and segment a target instance across space and time given a referring expression. To tackle this, we propose ST4R-Splat, the first framework for STRS-4DGS. Specifically, our framework incorporates an Instance-Aware 4D Gaussian Referring Field that assigns time-invariant embeddings for robust spatial grounding, and an Instance-Level Temporal State Mapping module that enables view-independent temporal localization directly in feature space. To provide rich spatio-temporal semantic supervision, we develop an automatic, MLLM-based captioning pipeline that generates decoupled spatial and temporal textual descriptions. Evaluated on our newly constructed STRS-4DGS benchmark, our method significantly outperforms adapted state-of-the-art baselines across both timeagnostic and time-sensitive metrics, establishing a strong foundation for language-driven 4D scene understanding.