Slightly off-topic: is there anything that is as or more powerful than RPi4 but has more/expandable memory (say 16G or more) and still works with mainline kernel/distributions? In other words, something that I could setup as a build machine for CI?
If you're doing it just for the novelty or really want to do it on site natively at a reasonable cost check out the Jetson TX2 dev kit. The TX2 has been mainlined for a while now and this kit offers 8 GB RAM with a fast CPU and the ability to connect fast SATA/NVMe storage. While most distros would run on it you would probably need to make your own install package (or just run container images for builds if you don't NEED to have an exact kernel)
Mainline Kernel – work in progress
UBoot – work in progress
UEFI – work in progress