Hello Flavio, 

You can try a logic like this:

function onLoad() {

    var my_category = g_form.getValue('category').length;
    var my_cmdb_ci = g_form.getValue('cmdb_ci').length;
    var my_assignment_group = g_form.getValue('assignment_group').length;
    var my_short_description = g_form.getValue('short_description').length;

    if (my_category == 0 && my_cmdb_ci == 0 && my_assignment_group == 0 && my_short_description == 0) {
        g_form.addInfoMessage('Four fields are empty.');
    }
}

Please mark my response as Correct or Helpful, if you find it appropriate. 

________________________

Carlos Camacho
https://www.linkedin.com/in/camachojunior