Is there an extension available for OpenGL that allows for R4G4 texture formats?
It would be super useful for rendering tilemaps with a tilemap texture and a 16x16 tile tileset texture.
There is no such OpenGL format or extension providing one. Surprisingly, Vulkan has such a format, but only NVIDIA and AMD platforms (and ARM on mobile) actually provide it.
You can of course emulate one using GL_R8UI and some shader logic to extract the bits and manipulate them. You will of course need to implement filtering logic yourself.
It’s not clear what you mean by that. I don’t see what the side of a tilemap’s tiles has to do with the image format used for its pixels.
You could of course use GL_LUMINANCE4_ALPHA4, from 1996’s EXT_texture / GL 1.1. Presumably, any implementation which stores R4G4 tightly packed will do the same for L4A4.