Been thinking about this car and the mix of rumors we've heard... I think there is a chance the new 2.X turbo 4 cylinder could debut on the IS after all. When I heard "carryover engines," it was because the names did not change and there were no new trademarks like "IS 400" or something. To our point in the NX thread, Lexus is probably using "350" for the new turbo 4 cylinder, so an "IS 350" might not necessarily mean carryover V6.
With that in mind, I could be wrong and overly optimistic, but what I am thinking is: there will be two models, but three engines on the 4IS.
1) IS 300h: easy to assume with 2.5L hybrid 4 cylinder, ~220hp
2) IS 350: core IS model with new 2.X turbo 4cylinder and ~300hp/325lb-ft of torque
3) IS 350 F Sport: Sport model with carryover GR V6, 311hp/277lb-ft of torque (F-Sport comes only in V6)
Does this seem stupid? Yes. But I honestly think Lexus will say, "Oh, sporty shoppers will want to have a V6 and it's more powerful on paper, so we can say we offered one" not thinking those same buyers will realize the turbo 4 has more torque, nearly the same horsepower, and is a brand new design.
For global markets where the IS is offered, I cannot see them offering the 8AR-FTS into 2025 and beyond. If it's true that this IS is all-new and on a TNGA platform, I think it makes sense for them to drop a newer "core" engine into it and then they keep the V6 for North America to satisfy who they think are sporty shoppers. Plus, if this new turbo 4 cylinder is to become Toyota's core workhorse engine, there is production efficiency in moving everything possible over to new TNGA engines and platforms. They still use the GR V6 everywhere (sedans, SUVs, trucks), so it's easy to just keep making that for a small subset of "sporty" buyers. 8AR-FTS is a lower volume engine and has been succeeded by 300h powertrain and this new turbo 4.