When support for Raspberry Pis has been added there was no model 5 yet. With the intention to only support the flagship the board configuration was added as rpi4b.conf. Since then support was extended across all models from 3 and up. However renaming a board config file is an issue. I already had the idea to address that but gave up on it: https://github.com/armbian/build/pull/7881
tl;dr: rpi4b image will work on all models from 3 to 5. https://www.armbian.com/rpi4b/
