This is likely that you write an old u-boot-spi image to the SPI, which only support GPT partitions. The Armbian images uses mbr partition. We have added the both mbr and gpt support in the latest u-boot-spi image.
https://dl.radxa.com/rockpi4/images/loader/spi/uboot-trust-spi-20190816_1128.img
The rockpi4b-rk-u-boot-latest package already contains that.
You can hook up a serial console to get the detail of what happened during the boot.