Well I just had a 'lightbulb' moment, and though I'd ring LU to find out.
Rang up their specific customer number Piccadilly Line .. asked when T4 was
going to close.

The chap politely told me that it had to close for construction work that
was abot to take place on the T5 station.

"I know that! but WHEN is T4 going to close?"

"Ah.." he says .. "I'll go and find out", put me on hold for 17 minutes and
then the phone went dead.

Ah well...