<wj-combo-box
  #theComboString
  id="theComboString"
  [itemsSource]="strings"
  [isEditable]="true"
  (initialized)="initCombo(theComboString,$event)"
></wj-combo-box>

 initCombo(combo, event) {

    combo.inputElement.maxLength = 4;

    combo.inputElement.addEventListener("input", function () {
      if ([...combo.inputElement.value].length > 4) {
        // Remove extra text.
        combo.inputElement.value = combo.inputElement.value.substring(0, 4);
      }
    });
  }