Deleting Multiple Bullets after Set Time

I am making an FPS game. To keep the game running fast, I decided to delete shot bullets after about 4 seconds. I decided to do that rather then on collision, because i was worried that the bullet would just pass through objects. I am just having trouble accessing the bullet clones. I tried attaching a delete script to the bullet prefab. It worked great, but only when I put the prefab in the project panel, not when I shot the bullet. So…what do I do. Can you point me in a certain direction? Thanks for all the help!

If you attach a script like this

function Start() {
    Destroy(gameObject, 4);
}

to the bullet prefab, it should delete every bullet consistently after 4 seconds.