Not sure if it will be, but if something like the Alstom (Bombardier) Aventra is ordered in similar style to class 710 units with 710/1 - 2 as 4 car units and 710/3 as five car units, then you could be using some of the 5 car trains as the Aventra have longer carriages to be replacing more than 1 unit of the class 465 fleet. You have to remember including the stored units there is 147 class 465 units and 36 class 376 units.
Now there are 19 class 465 units stored, which means that you are probably only really replacing 128 class 465 units. So you if replaced the class 465 units, with 4 car units you would be needing 512 carriages. I suspect though, they are probably looking at replacing both class 465 and the 36 class 466 units with as
@XCTurbostar stated in November last year, 5 car units.
I don't believe that as yet South Eastern are looking into replacing the class 376 fleet.