I have read some threats with the same issue, armbian images are not booting, there is not HDMI signal when turn on rockpi4. General solution is trying armbian image stored on radxa server https://dl.radxa.com/rockpi/images/third-party/Armbian_5.67_Rockpi4b_Debian_stretch_default_4.4.154_desktop_20181210-gpt.img.gz this armbian version works fine but it's too old, also i'm interested to use latest armbian-ubuntu-bionic
My case: I have a new rockpi4b 1.4v, not eMMC module, I have flashed several armbian versions on sdCard :
.. and old versions archived
None of them works! the issue is the same, not HDMI signal. I can boot fine using radxa images (armbian stretch and others OS stored there). Is possible something wrong on latest rockpi4 hardware and need a fix on armbian/uBoot configuration?