How do you protect a Nintendo DSi from a 4 year old?

For the outside case, there's the "Nerf Armor"; it exists for the DS lite, DSi and DSi XL.

With that it should resist almost any fall...

My daughter has played with my DS (not a DSi, but they're pretty much identical physically) since she was 3 and my DS is still in great shape.

They're pretty well constructed - a simple drop by a kid onto the floor doesn't really faze 'em. Obviously throwing it or something would be bad, but hopefully the kid will understand that's a no-no.

The touch screen also seems more than able to accept the attention of a 3 or 4 year old whose touch with the stylus may not be as kind and precise as an adult's. I've certainly talked to her about what not to do, but any rate my DS has taken its treatment well.

Who am I kidding? At this point it's her DS.

This insurance serves you ... is a protector.

http://www.amazon.com/DSi-Protector-Nintendo-Clear-DS/dp/B001U0O6KO

Obviously not become immune to the DSi but protects well.