Brew Bama had a chart posted on Fermentis yeasts a while back (really cool color graphics) and the relative sugars that each yeast fermented or didn't - S-04, being a British Ale Strain (right?) was not fermenting maltotriose, so it leaves a residual gravity a few points higher than some others (lager yeasts, for example, but especially the Belgian strains). I don't know if that aspect of S-04 might have or has an effect on the production/reduction processes of diacetyl, but maybe so? I am relatively blind to diacetyl, so I get it from mouthfeel and work backward.
Like sulfur, I think diacetyl will subside with time. I don't know the fermentation pathways and what byproducts they leave in beer, but maybe it is worth thinking about with S-04?
This speculation has nothing to do with real science - I don't do much science. Just thought it might be worth bringing up.