You can call it all in one function, like this:
<script type="text/javascript">
// Prevent variables from being global
(function () {
var div = document.createElement('div'),
ref = document.getElementsByTagName('base')[0] ||
document.getElementsByTagName('script')[0];
div.innerHTML = '­<style> iframe { visibility: hidden; } </style>';
ref.parentNode.insertBefore(div, ref);
window.onload = function() {
div.parentNode.removeChild(div);
var el1=document.getElementById("frameContainer");
el1.innerHTML="<iframe src=\"http://energyamplified.co.za/animation_medium.html\"></iframe>";
var el2=document.getElementById("frameContainer_medium");
el2.innerHTML="<iframe src=\"http://energyamplified.co.za/animation_medium.html\"> </iframe>";
var el3=document.getElementById("frameContainer_small");
el3.innerHTML="<iframe src=\"http://energyamplified.co.za/animation_small.html\"> </iframe>";
var el4=document.getElementById("frameContainer_very_small");
el4.innerHTML="<iframe src=\"http://energyamplified.co.za/animation_very_small.html\"> </iframe>";
};
})();
</script>
By the way, you are missing some ";".