I've always plugged my iPhones in at night (since the original in 2007), haven't had a problem with it. Batteries are always going to deteriorate over time, but mine didn't seem to get that much worse (usually kept my phones for 2+ years). I don't know about the 4S, but I don't see why it would be any different. If you're going to actually use your iPhone instead of having it on standby all the time, you probably need to charge it every day anyway, plugging it in at night while you sleep is the most logical way to do it, and the easiest to remember.