I’m guessing they stick with the 4 bangers in the LC, and leave the TTV6 for the GX to differentiate. Look at the grand highlander/TX; TX at least has the option for the top of line V6 hybrid, and the Highlander is capped with 4 banger power. With the GX only having been announced with one engine, I would think they would they would need to keep that exclusive to the GX, at least for now.
My guess remains that the new 4R gets the Taco 4 cylinder and the new LC gets the same GX TTV6. For all intents and purposes, in regards to marketing and maybe customer base, Toyota and Lexus are two different companies.