Important Note:
I encountered difficulties while trying to use the sorted_for plugin on my local machine, so I decided to proceed with testing my solution using a standard for loop instead.
Step 1
If you are unable to utilize forloop.index due to filtering out certain pages, you will need to manually count the loops by assigning values to a variable using assign.
The code provided below lists your pages with an accurate loop iterator (counting only the pages that are actually listed):
{% assign loopindex = 0 %}
{% for page in site.pages %}
{% if page.type == 'project' %}
{% assign loopindex = loopindex | plus: 1 %}
<div class="span3">{{ loopindex }} {{ page.title }}</div>
{% endif %}
{% endfor %}
Step 2
You must display <div class="row"> at the start of every first row and </div> at the end of every fourth row.
To determine the first and fourth rows, you can make use of the modulo function:
{% assign loopindex = 0 %}
{% for page in site.pages %}
{% if page.type == 'project' %}
{% assign loopindex = loopindex | plus: 1 %}
{% assign rowfinder = loopindex | modulo: 4 %}
<div class="span3">{{ loopindex }} {{ rowfinder }} {{ page.title }}</div>
{% endif %}
{% endfor %}
rowfinder will cycle through the sequence 1, 2, 3, 0.
Step 3:
Show <div class="row"> when rowfinder is 1, and </div> when rowfinder is 0:
{% assign loopindex = 0 %}
{% for page in site.pages %}
{% if page.type == 'project' %}
{% assign loopindex = loopindex | plus: 1 %}
{% assign rowfinder = loopindex | modulo: 4 %}
{% if rowfinder == 1 %}
<div class="row">
<div class="span3">{{ page.title }}</div>
{% elsif rowfinder == 0 %}
<div class="span3">{{ page.title }}</div>
</div>
{% else %}
<div class="span3">{{ page.title }}</div>
{% endif %}
{% endif %}
{% endfor %}
Step 4:
One final detail remains: when the total number of pages is not a multiple of 4, there will be a missing </div> at the end.
If the number of pages is divisible by 4, the last value of rowfinder will be 0.
Therefore, you simply need to display the closing </div> when the value of rowfinder is anything other than 0.
Add this line after the for loop to address this issue:
{% if rowfinder != 0 %}
</div>
{% endif %}
...and that concludes the process!