[DOC] Update Gemma 4

[jeejeelee]

Purpose

Test Plan

Test Result

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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--39661.org.readthedocs.build/en/39661/

[gemini-code-assist]

Code Review

This pull request adds Gemma 4 to the supported models documentation. The review feedback identifies a formatting error where a leading pipe was omitted, suggests using the text-only architecture name for this specific table, and recommends correctly indicating LoRA support.

[gemini-code-assist]

The entry for Gemma 4 has several issues that should be addressed:

1. Formatting: The line is missing the leading pipe (|), which breaks the markdown table rendering.
2. Architecture Name: This table is for text-only generative models. The correct architecture name for the text-only version is Gemma4ForCausalLM. Gemma4ForConditionalGeneration is the multimodal variant and should be listed in the Multimodal Language Models section instead.
3. LoRA Support: The implementation in vllm/model_executor/models/gemma4.py indicates that LoRA is supported (it inherits from SupportsLoRA). The LoRA column should be marked with ✅︎ to reflect this capability.

Suggested change

	`Gemma4ForConditionalGeneration` | Gemma 4 | `google/gemma-4-31B`, `google/gemma-4-26B-A4B-it`, etc. | | ✅︎ |
	| `Gemma4ForCausalLM` | Gemma 4 | `google/gemma-4-31B`, `google/gemma-4-26B-A4B-it`, etc. | ✅︎ | ✅︎ |

[DarkLight1337]

There is already a PR #39607