Since the GN4 version 2.4. you can use the autoComplete in i4 search for categories.

      <searchForm>

        <ssc:and>

          <ssc:fullTextBox path="nav:text" label="Full Text" operators="ContainsAll ContainsSome Contains Equal Near StartsWith EndsWith Advanced"/>

          <ssc:textBox path="@name" operators="Contains Equal StartsWith EndsWith" />

          <ssc:treeObjects path="gn4:folderRef/@idref" label="Folder" operators="In InSubtree NotIn NotInSubtree"

            attributeFullName="folderObject.folderRef" selectionMode="Multiple" refAttributes="folderParent" refObjectType="folder" showHomeFoldersOnTop="true" />

          <ssc:calendarTime path="@modifiedDate" label="Modified Date" operators="On Today Yesterday Before After LastDays LastHours LastMinutes LastMonths LastWeeks LastYears Between"/>

          <ssc:calendarTime path="@creationDate" label="Created Date" operators="On Today Yesterday Before After LastDays LastHours LastMinutes LastMonths LastWeeks LastYears Between"/>

          <!--<ssc:autoComplete path="gn4:cats/gn4:ref/@idref" attributeFullName="article.cats"/>-->

          <ssc:autoComplete 

                                         path="gn4:cats/gn4:ref/@idref" 

                                         attributeFullName="article.cats" 

                                         searchAttributes="name, description"/>

        </ssc:and>

      </searchForm>