What does the 4bpp mean? 4 bits per pixel? If so, what's the pixel format? 1111?
Or it means 4 bits per pixel-component? i.e 4444.
Texture.defaultTo4bpp defined the pixel format of textures in GPU, right?
Yes, it's the format used by the GPU. I either means 4444 or 5551. The engine decides which one you use based on the texture.