A “No” to both questions.
Just the original wording is slightly ambiguous:
It means that the only “official” way to obtain a root partition of 4 GB on an Xperia X, Xperia XA2, Xperia 10 and Gemini PDA is by flashing SailfishOS ≥ 4.6.0.
If one simply upgrades to SailfishOS ≥ 4.6.0 the root partition size will not be altered.
References: First message in “Sailfish Community News, 21st March 2024” and almost half of this discussion thread “Community meeting on 14th March 2024”.