Of the two openings on the bottom of your 4S... only one is the speaker. The other is the microphone. As far as the speaker being "easy" to blow out... I'd say no. However, if you're listening to music with ultra low bass frequencies at the highest possible volume... then it's quite possible that you'll damage the small driver.
It's doubtful that you'll blow the speaker out with Apples normal volume limits..
Now I can't speak for those who edit their music to be played at the maximum volume..
I have jacked my speaker volume up to ridiculous levels via file modification and my speakers have never blown or sounded out if tune in any way. Still don't.