Porn and sexual norms have been inextricably linked for decades now. One need only consider the once shocking, now common practice of pubic hair removal to understand this. That cat is out of the bag, so to speak.
And not all porn is like the porn you're describing. Erotic literature and comics are quite popular with the younger generations, which manage to depict outlandish fantasies and fairly normal sex within caring relationships with surprisingly similar frequency. Not to mention, if you consider things like crossdressing and transexual porn, one person's kink can be another's lifeline. To censor all porn, apart from being reprehensible on the grounds of freedom of speech, is throwing the baby out with the bathwater.
As for 4 year olds, I actually don't think they need to be worried about. I grew up with the internet, so naturally I stumbled across porn a few times before I hit puberty. I remember being quite surprised at first, but I got over it pretty quickly. I said "gross," clicked back, went back to surfing the web and playing video games, and never really thought about it or sought it out again until the hormones kicked in. Of course that's an anecdote, but I think finding porn as a kid is about as inconsequential as walking in on your parents having sex.
What people should be fucking terrified about is the popularity of camera and GPS-equipped smartphones that your naive kid can use to post geotagged selfies up for the entire world to see, making it trivial for predators to stalk them. But few parents are aware of the danger of this, and a firewall wouldn't help it anyway.