You should be OK. There is going to be a lot of yeast in a packet of S-04 anyway. For rehydration (and I'll probably get flamed for this), I usually use a bottle of room temperature (pre-sealed) drinking water. Our water supply sucks so we buy those flats of .5 liter bottles for drinking. I usually just sit one out first thing in the morning on brewday so that it has time to come to room temp while I am doing my other stuff.
I have a 500ml flask that I use for rehydration. During the boil, the flask gets a StarSan bath. When I put my chiller in the wort, I remove the flask from the StarSan then open the water bottle and give the neck a quick swab with an alcohol-soaked cotton ball. I put about 200 ml of the room temp water into my flask and pour in the dried yeast (the outside of the yeast pack also gets a wipe with the cotton ball.
Aluminum foil goes over the mouth to the flask. About 15 min later, I give it a swirl to mix the remaining dried yeast and water.
When my wort is chilled, transferred and aerated, the foil comes off and the lip of the flask gets a swipe with a new alcohol-soaked cotton ball. I give it about 30 sec or so to air dry, then into the fermenter she goes.
Many a clean fermentation has started this way at my house.
If my OG calls for more than one 11g pack of yeast, the flask just gets more water and more yeast.