How to make 1 or more of total 4 fields mandatory
You have to implement your own validation logic through a hook_form_alter().

    &lt;?php
    function MY_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id){
      if ($form_id === 'YOUR_FORM_ID')
        $form$form['#validate'][] = 'YOUR_VALIDATION_FUNCTION';
      }
    }

    function YOUR_VALIDATION_FUCNTION($form, $form_state){
      $assingned_values = 0;
      if(!empty($form_state['values']['FIELD1']['und'][0]['value'])){
        $assigned_values++;
      }
      if(!empty($form_state['values']['FIELD2']['und'][0]['value'])){
        $assigned_values++;
      }
      if(!empty($form_state['values']['FIELD3']['und'][0]['value'])){
        $assigned_values++;
      }
      if(!empty($form_state['values']['FIELD4']['und'][0]['value'])){
        $assigned_values++;
      }
      if ($assigned_values === 0){
        form_set_error('', 'You must set at least one field');
      }
    }