Orderby writing sequence
The fixedOrder parameter makes your entries to be returned in the same order as the IDs you pass to the id parameter. So a simple solution would be to look up the entry IDs of your single section entries and get your entries via their IDs instead of the section handles.

{% set entryIds = [3, 14, 15, 9, 26] %}

{% for entry in craft.entries.id(entryIds).fixedOrder(true) %}
    &lt;li&gt;&lt;a href="{{ entry.url }}"&gt;{{ entry.title }}&lt;/a&gt;&lt;/li&gt;
{% endfor %}


You can eighter look up the IDs in the database, or probably faster: add {{ entry.id }} to your current nav loop!

Update: If you have a good reason to get them via their handles, use this snippet to get the entry IDs.

{% set sectionHandles = ['arbeiten', 'info', 'kontakt', 'shop'] %}

{% cache globally for 3 years %}
    {% set entryIds = [3] %}
    {% for handle in sectionHandles %}
        {% set entryIds = entryIds|merge([ craft.entries.section(handle).first().id ]) %}
    {% endfor %}
{% endcache %}