Yes it is; journeys between the three Heathrow stations are free, even if you travel via Hatton + (ie. a journey from T123 to T4). Customers who touch in or out at Hatton + however will be charged the appropriate fare.Is this journey free?
On my Oyster card, the journey cost appears to have been zero.
By contrast, physical tickets cost actual money.
All buses in the area are free too as well as the Heathrow Express when travelling between terminals.