var flat = [
{ id: 1, parent_id: null }, 
{ id: 2, parent_id: 1 }, 
{ id: 3, parent_id: 1 }, 
{ id: 4, parent_id: 2 }
];

function searchChildren(parent_id) {
    var result = [],
        i;
    for (i = flat.length - 1; i >= 0; i -=1) {
        if (flat[i].parent_id === parent_id) {
            result.push(flat[i]);
        }
    }
    return result;
}

function recur(item) {
    var result = { id: item.id, parent_id: item.parent_id, children: [] },
        items = searchChildren(item.id),
        i;
    for (i = items.length - 1; i >= 0; i -=1) {
        result.children.push(recur(items[i]));
    }
    return result;
}

alert(JSON.stringify(recur(searchChildren(null)[0])));