And one last mail, here is a log from Windows 7 on my Radeon X1600 (r500) machine. This was the GPU where AMD originally introduced fetch4, and the result looks identical to the newer HD 5770 (r700 I think). 6 fewer failures, I assume a depth format is not supported.

https://www.khronos.org/registry/OpenGL/extensions/AMD/AMD_texture_texture4.txt may also be an interesting read. The other interesting side-note is that r500 does not support unconditional NP2 textures, so we'd have a case where FETCH4 could be used with GL_ARB_texture_rectangle, but I seriously doubt that this is worth the effort because this hardware is more than 10 years old. I doubt mesa exposes AMD_texture_texture4 on the r300g driver.