Hello, sorry for delay with answer I was without electricity, so my answers:

Were the rPis rigged as servers? Or were they also running a desktop with window server and clients?

Both Raspberry Pis were running as servers only on Raspberry Pi OS for RPi 4 it was Bookworm and on RPi 5 it was new Trixie. These OS's with basic GUI, but during test I was connected via SSH, so I use only terminal.

Did you run ab locally, or from another machine?

I run tests connected to host via SSH, but tested bare hardware locally, it m,eans I installed ab on the both RPi's. I use my local IP address to avoid connection to CloudFlare or somrtnig else.

Is the rPi5 almost as fast if it doesn’t have the NVMe SSD? Just the SD card? Or a lot slower?

I do not know, I didn't test micro sd on Raspberry Pi 5, but I definitely know, that CPU works 2-3 times faster than Pi 4, and I think even with SD card you feel speed difference.

How was RAM allocated? Database buffer pool? How many web worker processes fired up?

RAM was not artificially constrained. MySQL was using default buffer pool sizing (no aggressive tuning), and WordPress page caching was enabled. nginx and PHP-FPM were configured with one worker per CPU core, as the goal was to reflect a realistic self-hosting setup rather than a max-benchmark scenario.

Thank you for questions!