If you follow the index.html on Bartaz GitHub site, you would see, that you missed a line in configuring your presentation.
<script src="js/impress.js"></script> //add this line and edit the path accordingly
<script>impress().init();</script>
The code for rotating itself looks fine from my point of view. You just need to tinker with the positioning and the x and y axis.
Kind regards
//edit:
Check your data-rotate
in your third and fourth step. the -
is missing, and therefor it does not work, like it should.
<div id="impress">
<div id="1von4" class="step slide" data-x="0" data-y="0" data-rotate="0">
<p style="font-size:36px;">This is my first slide.</p>
</div>
<div id="2von4" class="step slide" data-x="950" data-y="950" data-rotate="90">
<p style="font-size:36px;">This is my second slide.</p>
</div>
<div id="3von4" class="step slide" data-x="0" data-y="1800" data-rotate="180">
<p style="font-size:36px;">This is my third slide.</p>
</div>
<div id="4von4" class="step slide" data-x="-950" data-y="950" data-rotate="270">
<p style="font-size:36px;">This is my fourth slide.</p>
</div>
</div>