This seems similar to the Scottish Heavy I just made awhile back, it was 1.084 and after 10 days of primary holding 1.028 I figured it was done. Racked it over to secondary to age it, and the the rest of the yeast flocculated out and finished fermenting. S04 is high flocculation with a rated attenuation of 75% and from 1.080 should get you somewhere near 1.020 @ 75% so might be just a sit back and wait, if you can secondary it and put it in a closet if all else fails.