New LCD would do the same to LCD as the sensor (PEN-F, which has been made already). And not sure does Olympus want bigger cameras throughout the range with big LCD. Bigger LCD is more expensive. New UI need developing and funds.You don't at the sensor. You do at the body. No sealing, less buttons, smaller EVF. Drive the UI to mostly touch-screen and stick a monster LCD on it (like the X-A7's 3.5" touchscreen) or shrink the size even more so the back is mostly screen (a la M200) with a normal 3" screen (E-PL there). Stuff the card in the battery hatch instead of a separate card door, reduce the interface (they could get away with only a single USB-C port, using that for everything). Drop the charger for pure USB charging (Sony's done that). Lots of places they can trim to keep the bodies differentiated while making the E-M10IV both cheap & decent.
Oly's got low enough in terms of volume that if they can get everything in production on one sensor they should actually get noticeable savings out of that, just because their volume across 2-3 sensors is not high enough to keep pricing on the sensors low.
Battery hatch has the card already in my E-M10 II. Personally I want separate charger so I can recharge at the same time as I take photos. Of course it can be sold separately.
Is less buttons or weather sealing big enough differentation? Likely they keep the E-M10 III IBIS there.
Does anyone know how customisable is the wheel with modes on top of E-M5 III? E-M1 II has more C-modes, but can you change those other mode functions in E-M5 III? Its possible in E-M10 II.