What are valid paths to apply new features to SAP S/4HANA 2021 without changing the base release?

To apply new features to SAP S/4HANA 2021 without changing the base release, updating to SPS04 of release 2021 is a valid option. The Stack Packs (SPS) refer to the Support Package Stacks, which are a collection of improvements and corrections that can be applied to an existing software version. Each new Support Package Stack contains essential updates, including new features, enhancements, and bug fixes.

When you update to the SPS04 of release 2021, you gain access to new functionalities introduced between the earlier Support Package Stack and the SPS04. This allows organizations to take advantage of enhancements in SAP S/4HANA while maintaining the same base release, which is vital for ensuring stability and consistency in system environments.

The other choices involve pathways that either change the release or are not valid updates for the context of applying new features without altering the base version. Thus, the selected answer reflects the mechanism designed for updating an existing installation specifically within the same major version framework.