Terminal 4 would be in the way of runway four surely?
I'd have thought that a runway going through the T4 site would be too close to 09R/27L to allow sufficient separation for independent operations.
A parallel southern runway with the same separation as between the two existing runways would run just to the north of the Esso fuel farm and then neatly bisect Stanwell and the northern of the two Staines Reservoirs.