{% assign featured_ids = '' %}
Featured:
{% for post in taxonomy['tags']['featured'].posts limit:4 %}
{% assign featured_ids = featured_ids | append:',' | append:post.id %}
<article class="featured">{{post.title}}</article>
{% endfor %}
Everything else:
{% for post in posts %}
{% unless featured_ids contains post.id %}
<article class="normal">{{post.title}}</article>
{% endunless %}
{% endfor %}