Using Software Rendering to make an External GPU
Theoraticaly this can work, but I am not sure if it would be very efficient. Modern GPU's are tailored for doing low level graphics operations and the drivers that manage them are also tailored for efficiently overseeing these operations to render your scene.
Now you can ask all the hardware questions related to using Raspberry Pi, how to access its cpu etc to Raspberry Pi SO.
As per software side, I think P. Shirley's books on
ray tracing should get you started on rendering on cpu's rather than gpus. Using multiple threads is extensively discussed in the following issue of the source code of the book. So once you had written your single threaded cpu ray tracer, you can adapt it easily to execute in multiple threads.

Update 21-01-2021
I noticed that there is already a driver for Raspberry Pi 4 provided by mesa, which gives you access to OpenGLES 3 functionality. I think it is a decent trade off, I was even surprised to see that there is actually a graphics driver for Pi.
Unless you insist on ray tracing your scenes, I would suggest to use that for your experiments.