Hi!
My son is also 4 and a half and we are going through this, too. I think that things people have already mentioned (a child seeing or hearing about some kind of scary event) can trigger some of this.
But I think also some of it is children's cognitive development. They have a better understanding that bad things can happen. My daughter also went through a phase like this. (She is now 7--she occasionally still has periods of anxiety, but I can usually figure out what's going on and talk with her about it.)
But it is impossible to protect our children from every single thing. (The bath water still goes down the drain. The idea of going down the drain is in his head. But I can keep reassuring him that he will never go down the drain until that anxiety passes.) People do die. There are accidents. We do have to keep teaching our children about safety.
I'm trying to be reassuring, avoiding exciting movies near bedtime, and I will go with him to the bathroom (since it's on the 2nd floor). But I am trying not to "coddle" the fears too much. (Sometimes it can be trying...)
Here is a site that has some helpful advice: http://www.keepkidshealthy.com/parenting_tips/fears.html .
I'm waiting to see whether this fear abates over the next 2 months or so. Depending on how long it goes on or how serious it becomes, I would consider counseling. But I'm still in a wait-and-see-and-be-patient stage.
Good luck!