The Raspberry Pi 4 has a 64 bit architecture, but up till recently, all Raspberry Pi software builds (including for the Raspberry Pi 4) have been 32 bit. There are now versions of a number of 64 bit operating systems that will run on a Raspberry Pi 4, but they are all experimental.
All Portsdown builds are based on the 32-bit Raspios Buster Lite operating system.
The Portsdown 2020 build runs on the Raspberry Pi 3 hardware. The Portsdown 4 build runs on the Raspberry Pi 4 hardware.
I have no plans to move Portsdown to 64 bit software.
More details on the Wiki here: https://wiki.batc.org.uk/The_Portsdown_ ... _Evolution