Terminal 4 will begin operations on 31 October 2017.
http://www.changiairport.com/corpora.../newsroom.html
Changi Airport Group (CAG) today announced that the new Terminal 4 (T4) at Changi Airport is scheduled to commence operations on 31 October 2017.
To ensure a smooth transition of flight operations from the existing terminals, the operations of the nine T4 airlines – the AirAsia Group (of four airlines), Cathay Pacific Airways, Cebu Pacific Air, Korean Air, Spring Airlines and Vietnam Airlines – will be shifted to T4 over one week on the following dates:
Tuesday, 31 October 2017 - Cathay Pacific Airways and Korean Air
Thursday, 2 November 2017 – Cebu Pacific Air and Spring Airlines
Tuesday, 7 November 2017 – AirAsia Group and Vietnam Airlines
The first arrival and departing flights at T4 will be operated by Cathay Pacific – CX659 from Hong Kong (STA[1]: 0540 hours) and CX650 to Hong Kong (STD1: 0650 hours) respectively.