I have an array of HTML elements. I'm checking whether a previous object in the array exists like this:

```
var boxes = $('.box'); // creating the array

boxes.each(function(i){ // going through the array
    var prevBox = i>0?$(boxes[i-1]):false; // check whether a previous box exists
    if (prevBox) { } // do something
    else { } // do something else
});

```

This works well. But I would also need to check the existence of every fourth object (box) in the array, or more precisely whether an object three objects before the current one exists.

This doesn't work:

```
var prevBox = i>0?$(boxes[i-4]):false;

```

I believe using `jQuery.grep()` and checking if `(i % 4) == 0` might be the answer, but with my limited knowledge of Javascript, I don't know how to apply it to what I have now.

Anyone can help? Thanks!