$(document).ready(function()
{
    changeBG();
});

function changeBG()
{
    var ran_unrounded = Math.random() * 4;
    var ran_number = Math.floor(ran_unrounded) + 1;
    $("body").removeClass().addClass("body"+ran_number);
    timeoutobj = setTimeout(changeBG, 10000);
    return false;
}