I’ve never been to Heathrow T4 so I don’t know the layout but I assume you’d need to tap out of the Eliz. line station and into the Picc. Line station?
If not, the question wouldn’t have meaning since the system wouldn’t know you’d been via Heathrow.
If so, I’d be very surprised if there was any sort of out of station interchange functionality that joined your journeys together as Heathrow isn’t an interchange station by any set of criteria. On that basis you’d surely be charged for a journey to Heathrow T4.