I've never had it down that low, but if you pitched huge it might work there. I've had it down to about 44 or 45. If you had fermentation started spending some time at 40 won't make it quit, but I wouldn't keep it at that temp.
i pitched at 60. trying to keep it at 55 degrees just when i add the ice the water drops down to 40 but it goes back up to 55. im going to let it stay at 55 for a dr. tomorrow!!!
You are probably safe taking it down to 40 with ice as often as you can; the actual fermentation temp will not be anywhere close to there. As long as you keep seeing good evidence of activity keep doing what you're doing.