I am brewing with fermentis S-04 and from experience (three years ago) I know that this yeast is slow.
This weekend I brewed a small batch and today the attenuation is 83% (according to the yeast's specification it should be 75% at most) only 4 days have passed.
I suspect it's contamination, could it be something else?
Edit: Its a 18 liter (4.7 gal) batch and the recipe is 5Kg (11 pounds) of 6-rows base malt and 1 Kg (2.2 pounds) of cane sugar
Note: I brew in México's Michoacan state, my temp ranges from 15 to 26