You need to wrap your form processing code in a conditional so it doesn't run when you first open the page. Something like so:
if($_POST['num1'] > 0 && $_POST['num2'] > 0 && $_POST['num3'] > 0 && $_POST['num4'] > 0){
$itemQty = $_POST['num1'];
$itemCost = $_POST['num2'];
$itemSale = $_POST['num3'];
$shipMat = $_POST['num4'];
$diffPrice = $itemSale - $itemCost;
$actual = ($diffPrice - $shipMat) * $itemQty;
$diffPricePercent = (($actual * 100) / $itemCost) / $itemQty ;
}