提出四角色框架，解析大模型在科研中的定位与边界

Large language models (LLMs) are increasingly used in scientific research and discovery, supporting tasks ranging from literature retrieval and synthesis to hypothesis generation, autonomous experimentation, and research evaluation. Existing surveys often conflate scientific research with scientific discovery and typically organize systems by domain, task, or autonomy level alone. In this survey, we propose a four-role framework for understanding LLMs in scientific innovation: Assistant, Collaborator, Scientist, and Evaluator. The framework integrates three complementary dimensions: autonomy level, cognitive function, and scientific innovation, to distinguish research-oriented support from frontier-oriented discovery. We review representative methods, benchmarks, and evaluation practices for each role, examining their capabilities, limitations, and human oversight requirements. Across the literature, Assistant systems are comparatively mature in retrieval and synthesis but remain unreliable in open-ended applications; Collaborator systems expand the space of candidate hypotheses yet struggle with novelty-grounding trade-offs; Scientist systems increasingly automate research workflows but face reliability and safety bottlenecks; and Evaluator systems support review and verification while remaining weak in novelty assessment. We argue that progress in AI for science depends not only on model capability, but also on evaluation, oversight, accountability, and institutional integration.