How do you explain death to a four year old?

This afternoon I had to take JR to the doctors as he has impetigo on his face! As regular readers know I don't drive, that meant we had to walk although its not far just a straight road and only takes about 20 mins to walk there with the kids.
Today Jackson was enjoying the nice weather and taking a lot of notice of his surrounding, this was when he made a discovery in the middle of the pavement a dead robin! He crouched down to have a look at it and said "mommy it's a bird! Why isn't he in the sky flying with his friends?" I really didn't know how to answer his question so said he was sleeping having an afternoon nap like LA does.
The thing is I'm worried now that I've lied to him, and I should have told him the truth but he's only 4.5 I want him to keep his innocence a little bit longer!
Is that wrong of me what do you guys think??