Found the problem: They have removed the remote option in bootstrap 4
remote : This option is deprecated since v3.3.0 and will be removed in v4. We recommend instead using client-side templating or a data binding framework, or calling jQuery.load yourself.
I used JQuery to implement this removed feature.
$('body').on('click', '[data-toggle="modal"]', function(){
$($(this).data("target")+' .modal-body').load($(this).data("remote"));
});