If at all possible, try to get it down to 22 or lower. With ambient at 21, you're going to need ice to counter the heat from the fermentation. I put my fermenter inside my brew kettle and fill the kettle with ice water. The first two to three days are the most critical with S-04. Again, don't fret, it won't ruin your beer if you're higher, it'll just add more English ale fruitiness to the brew. I fermented an English Pale with S-04 this summer that got up to 25. You can definitely taste it in the beer, but since English Ales can have a bit more of that character, it's quite to style. I'd imagine a stout would do a better job of hiding it.