In article , Bondee wrote:

It's been a while since I used the Picadilly Line from Heathrow, but IIRC
you already have to go via Hatton Cross to get from T123 to T4.

That's correct. You can go the other way directly, though.

Niklas
--
"I didn't get a toy train like the other kids. I got a toy subway instead. You
couldn't see anything, but every now and then you'd hear this rumbling noise go
by."
-- Steven Wright