[Doc] Add Gemma 4 to supported models list

[z1ying]

Purpose

Gemma4ForCausalLM and Gemma4ForConditionalGeneration are already implemented and registered in vLLM (gemma4.py, gemma4_mm.py), but were missing from the supported_models.md documentation table. This PR adds the two missing entries.

Changes

Documentation only — docs/models/supported_models.md:

• Text-only table: add Gemma4ForCausalLM — LoRA ✅, PP ✅, example model google/gemma-4-E2B-it.
• Multimodal table: add Gemma4ForConditionalGeneration with modalities T + I⁺ + V + A*, PP ✅, no LoRA, example models google/gemma-4-E2B-it, google/gemma-4-E4B-it.
• New legend entry: introduced * as a generic marker meaning "Only specific variants of the model support this modality", reusable for future models with variant-specific modalities.
• New note block: clarifies that (a) audio input is only supported by the gemma-4-E2B and gemma-4-E4B variants, and (b) the model does not ingest videos directly, but vLLM's Gemma 4 implementation supports video inputs by handling video processing internally — users can send videos directly in the message structure.

Both Gemma 4 rows follow the same format as the existing Gemma 3 entries directly above them.

How the fields were verified

• LoRA / PP: checked class declarations and MRO in gemma4.py and gemma4_mm.py.
• Modalities: confirmed via discussion with the model author in this PR — text + image is universal across all variants; audio is gated on the presence of an audio tower (E2B / E4B only); video is not a native input but is achieved via an external video ingestor that feeds text + frames at 1 fps into the model.
• Example HF model IDs: match entries in tests/models/registry.py.

Notes

The additional * legend is intentionally generic rather than Gemma-specific so it can be reused by future models that have modality support varying across checkpoints within a single architecture.

Test

pre-commit run --files docs/models/supported_models.md

No source code was changed. Existing Gemma 4 tests are unaffected.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--39607.org.readthedocs.build/en/39607/

[gemini-code-assist]

Code Review

This pull request updates the documentation in docs/models/supported_models.md to include support for Gemma 4 models, specifically adding Gemma4ForCausalLM and Gemma4ForConditionalGeneration to their respective tables. I have no feedback to provide.

[z1ying]

Updated. Please let me know if this looks good now. @lucianommartins @ywang96

[ywang96]

Thanks for updating our doc!

[z1ying]

Thanks for the review! @ywang96 Could you help merge this PR?