To answer your intended question, probably not. Diffraction should not be an issue at f/4 or f/8. Depth of field will be different. Lens sharpness/eveness characteristicts may be a factor.
Now, you need to understand that: "F4 @ 1/1000TH and F8 @ 1/500 " are not equivelent exposures. The latter is one stop darker. So based on that, you would notice a difference. (But, I know that you made a mistake and really meant equivelents, no?)
I read an article stating a majority of cameras have a "Sweet Spot" for aperture, being around 8.0, anything higher could produce diffraction (Image Quality Loss), and recommended 8.0 was a good aperture for detail.
My question is, if you're in great sunlight, would you notice a difference between say..
F4 @ 1/1000TH and F8 @ 1/500
I'm just curious what the rule of thumb here is, or if it even matters. I do a lot of daytime shooting with a Canon 17-40 L and wanted to run that by this enourmous forum of knowledgable people