According to some leaked source code, Samsung's Exynos 5433 chip (the one that will run inside the international version of the Galaxy Note 4) will be an ARMv8/64-bit chip. That's the good news; the bad news is that the chip will be running in 32-bit mode only, at least for now.
The leak shows that the SoC will include four 1.3 GHz Cortex A53 cores, four 1.9 GHz Cortex A57 cores and a brand-new Mali-T760 GPU. The CPU is what we've come to expect from Exynos chips so far; it's a big.Little setup that uses the lower-end and more power-efficient Cortex A53 cores (which are successors to Cortex A7) for simpler tasks and the Cortex A57 cores (successors to Cortex A15) for more complex tasks.
The CPU is built on the 20nm process and also benefits from Cortex A50-series improved power consumption, so it could be much more efficient than some of the latest Exynos chips while also delivering higher performance.
The Mali-T760 GPU is ARM's latest and supports up to 16 cores, although according to the leak this one is likely to support only eight cores, each clocked at 700 MHz. (It wouldn't be the first time Samsung used fewer cores than the maximum allowed for its GPUs.)
The 16-core version might come in a future Galaxy S6 or Galaxy Note 5. ARM has said before that Mali-T760 has a peak performance of 380 Gflops, which would rival Tegra K1's 365 Gflops, but it's likely that only the 16-core version will achieve that kind of performance.
The new GPU also supports the latest OpenGL ES 3.1 graphics API. The chip's driver will likely support the Android Expansion Pack for enhanced graphics, too, once Samsung upgrades Galaxy Note 4 and other future devices using this chip to Android L.
While Exynos 5433 will benefit from some of the architectural improvements of the ARMv8 instruction set, it seems the chip will only work in 32-bit mode. ARMv8 includes both 32-bit and 64-bit modes, but Samsung appears to have chosen to use only the 32-bit one for now.
There are two theories for why the company might have done this. One is that the Galaxy Note 4 will arrive with Android 4.4 and not Android L, which is the only version of Android that supports 64-bit platforms. The other is that the U.S. version of the Galaxy Note 4 will only ever be 32-bit, because Qualcomm's Snapdragon 805 is an ARMv7 chip, and Samsung didn't want its U.S. customers to become frustrated that the Galaxy Note 4 model they are buying isn't 64-bit like the international one.
We'll see which theory is true when the Galaxy Note 4 receives the Android L update. If the international version continues to eschew the 64-bit mode -- despite fully supporting it after the update -- then we'll know Samsung just didn't want its U.S. customers to feel left out.