Edit in JSFiddle

.iframe-holder-1
{
    width: 100%;
}

.iframe-holder-2
{
    padding-top: 75%; //4:3
    position: relative;
}

.iframe-holder-2 iframe
{
    position: absolute;
    top: 0px;
    left: 0px;
    width: 100%;
    height: 100%;
}

<div class="iframe-holder-1">
    <div class="iframe-holder-2">
        <iframe src="http://labs.qnimate.com"></iframe>    
    </div>
</div>