To get precise all Pi4 are 64 bit (older ones might be also I don't know) however they have been releasing Raspbian only in 32 bit. It is only with the release of the 8GB version they had to provide a 64 bit version to use all the RAM.
Pretty complex in general. There is both 32bit and 64 bit Raspbian both of which should run on all Pi 4 models. Then there is Octopi 17 and 18 which are 32bit and 64 bit respectively. Then there is the Kernel issue which affects both versions of Octopi (well mostly the plugins - because running without plugins I think either Octopi 17 can be upgraded to Kernel 5.4. Octopi 18.1 is already on Kernel 5.4.
I'm not really 100% but from my limited understanding the Kernel issue is the biggest issue. It just happens that the 64 bit version of Raspbian comes with the latest Kernel. You can cause the same issues if you sudo apt-get upgrade on Octopi 17. Because it will update the Kernel to 5.4 while it is still 32 bit.
Even the differences I see (speed, file handling is super fast) could in theory be the new kernel or the 64 bit OS. When it is safe to sudo apt-upgrade my production server we could make that comparison.