Take a look here.
https://forums.lenovo.com/t5/Moto-G4-Moto-G4-Plus-Moto-G4/Moto-G4-Plus-32bit-Processor/td-p/3337312
And here.
http://forum.xda-developers.com/moto-g4-plus/how-to/sd-617-32-bit-t3382892
It seems Lenovo installed a 32-bit kernel.
Correct. You need the bootloader unlocked and a custom recovery installed.And of course without unlocking my bootloader I cannot install a 64bit one?
Correct. You need the bootloader unlocked and a custom recovery installed.
It seems the data bus is of 32 bit.
Who knows, but reading through those two links it looks like there's a big chance that it will stay as it is.Isn't it strange that they'd go with a 64bit processor and then limit it. Perhaps Nougat well bring us to ARMv8?