jQuery Ajax 404 Handling

Im trying to access a 404 event which I can see coming back as 404 with firebug but the error function is not kicking in, With my below code I always get Error: success ?.

ie.

$.ajax({
    type: 'get',
    url: 'url: 'https://admin.instantservice.com/resources/smartbutton/5702/10945/available.gif?' + Math.floor(Math.random()*10001),
    success: function(data, textStatus, XMLHttpRequest){
        console.log('Error: ' + textStatus);
    },
    error:function (xhr, ajaxOptions, thrownError){
        alert(xhr.status);
        alert(xhr.statusText);
        alert(xhr.responseText);
    }
});

Again I know 1000% that im getting 404 Not Found in firebug it never triggers the error.

Am I missing something ?