All GCN4 cards will cause a black screen on High Sierra full boot. There are several work arounds:

• You can use an additional external display or miniDP emulator dongle connected to the single working port.
• You can patch High Sierra
• You can enable the OpenCore agpdmod=vit9696 in the boot-args in the config.plist. This setting is not compatible with more recent macOS versions like Mojave and later, but if you do not use High Sierra OCLP will provide correct settings automatically.

Nevertheless OpenCore is the only way to go if you want or need to enable the HEVC and H.264 support of GCN4 cards.

The patch mentioned above is actually only a change to the Info.plist file as shown here. To disable SIP boot with Command+S+R, type csrutil disable and then reboot and let it boot with Command+S into single user mode:

% mount -uw /
% MYBOARD=`/usr/sbin/ioreg -l | grep board-id | awk -F\" '{ print $4 }' | grep Mac`
% cd /Systems/Library/Extensions
% /usr/libexec/PlistBuddy -c "Add :IOKitPersonalities:AppleGraphicsDevicePolicy:ConfigMap:$MYBOARD string none" % AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/Info.plist
% chown -R 0:0 AppleGraphicsControl.kext
% chmod -R 755 AppleGraphicsControl.kext
% reboot