[Repo Assist] docs: add 4.0.0-beta04 release notes entry

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Summary

Adds the 4.0.0-beta04 entry to docs/RELEASE_NOTES.md, covering changes merged since beta03:

• improve: surface schema description as XmlDoc on generated object and enum types ([Repo Assist] improve: surface schema description as XmlDoc on generated object and enum types (+5 tests, 348→353) #419)
• improve: add <returns> XmlDoc tag for operation response descriptions (+4 tests) ([Repo Assist] improve: add (returns) XmlDoc tag for operation response descriptions (+4 tests, 353→357) #420)
• test: add direct unit tests for XmlDoc module (+32 tests, 357→389) ([Repo Assist] test: add direct unit tests for XmlDoc module (+32 tests, 357→389) #422)
• eng: bump Microsoft.OpenApi and YamlReader 2.7.3 → 2.7.4 ([Repo Assist] eng: bump Microsoft.OpenApi and YamlReader 2.7.3 → 2.7.4 #416)

Test Status

Documentation-only change — no code changes, no build/test required.

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