When OpenCL is not available on Raspberry Pi 4, you can use OpenGL 3.0 ES compute shaders instead. Yeah, sure. But, driver issues aside, what really is the difference between OpenCL and OpenGL compute shaders? Mainly, the purpose of using OpenGL compute shaders is to integrate general purpose GPU compute into an existing, predominantly graphics application. OpenGL compute shaders have easier access to OpenGL state, but OpenCL needs state synchronization primitives and such.
Also, I found a little bit of useful information about how to use OpenGL compute shaders.
20200623/DuckDuckGo opengl compute shader
20200623/https://www.khronos.org/opengl/wiki/Compute_Shader
20200623/DuckDuckGo compute shaders versus opencl
20200623/https://stackoverflow.com/questions/15868498/what-is-the-difference-between-opencl-and-opengls-compute-shader