<input type="text" name="card_num" size="4" maxlength="4" onkeyup="return isNumberKey(this);">
<script>
function isNumberKey(e)
{
var numberRegex = /^[0-9]+$/;
if(!numberRegex.test(e.value))
{
e.value = e.value.replace(/\D/g, '');
}
}
</script>