Unfortunately it's also normal 4-year-old stuff. DS turns 5 this summer and he's been "suffering" from night terrors for years. I sometimes think we suffer more than him, though, because he remembers none of it in the morning (as long as we don't wake him).
DS is also a bit of a sleepwalker. Fortunately it's contained to his room at this point, but it is certainly unnerving.
I want to echo what has already been said. The night terrors (and now, the sleepwalking) are most prevalent when he's overtired or off schedule. We were on a mini-vacation this past weekend and stayed up late Saturday night. We didn't get any sleep because he screamed all night.
Very unsettling.
On a lighter note, when DS has prolonged night terrors, we bring him into our bed. We did this for a few nights, but he didn't recall how he got to our bed or why he was there in the first place. Finally he said, "Would you please leave me in my own bed tonight? I don't like yours!"
