We have made a pre-built openCV 4.0 package for ROCK Pi. Please refer: https://wiki.radxa.com/Rockpi4/Ubuntu
Basically,
sudo apt-get update
sudo apt-get install build-essential
sudo apt-get install cmake git libgtk2.0-dev pkg-config libavcodec-dev libavformat-dev libswscale-dev
and
sudo apt-get install python-dev python-numpy libtbb2 libtbb-dev libjpeg-dev libpng-dev libtiff-dev libdc1394-22-dev
sudo apt install libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev libavresample-dev
sudo apt install tesseract-ocr cmake-data liblept5
And finally.
sudo apt-get install opencv4.0