Existing tools still need abi3 there to install for the GIL-enabled builds. So for the short term, the alternatives are abi3 alone [^1], abi3.abi3t, or abi3.abi4. The first option prevents us from adding compatibility with 3.14t later; the latter two are cosmetic.
But there’s also the filename tags, which the current PEP doesn’t touch. Changing those to to abi4 would be an extra technical change (and a naming bikeshed: the files are still compatible with abi3 so they shouldn’t be just renamed, and we don’t have compressed tag sets here so we’d need to invent something like abi3-abi4.)
From a “branding” perspective, it would look weird if a abi3.abi4 (or abi4 only) wheel contained only .abi3.so files. Hopefully, an abi3.abi3t wheel with only .abi3.so files looks compatible enough.
IMO, the only thing that’s hamstrung is the naming :)