Hey magus, sorry about the delay.
(1) Download this JS file and save it to your modern/ template directory. Open it and edit the two var entries on first two lines to (a) match the backend comment limit number and (b) to match your language.
functions.js
(2) Open the modern index.html file and search for this codeblock:
<div>
<label for='tcomment'>{_MESSAGE}:</label>
<textarea name='tcomment' id='tcomment' cols='{formareacols}' tabindex="5" rows='10'></textarea>
</div>
...and replace it with this codeblock:
<div>
<script type="text/javascript" src="{template_live_site}/functions.js"><!----></script>
<span style='float:left;'>
<label for='tcomment'>{_MESSAGE}:</label>
<br/>
(<span id="jcpt_ticker"><!----></span>)
</span>
<script type="text/javascript">document.getElementById('jcpt_ticker').innerHTML=_JCPT_MSG_MAX_NUM+' '+_JCPT_MSG_MAX_TXT;</script>
<textarea name='tcomment' id='tcomment' cols='{formareacols}' tabindex="5" rows='10' onkeydown="jcpt_count(this.form.tcomment,this.form.jcpt_ticker);" onkeyup="jcpt_count(this.form.tcomment,this.form.jcpt_ticker);"></textarea>
</div>
Tested on 4.0 RC1 as OK