Shopify Liquid template that wraps looped items in a row on every third or fourth iteration.

_end_row.liquid

{% assign thumbs_per_row = settings.thumbnails-per-row | times:1 %}
{% if forloop.length > thumbs_per_row %}
  {% assign mod = forloop.index | modulo:thumbs_per_row %}
  {% if mod == 0 %}</div><div class="row">{% endif %}
{% endif %}

loop.liquid

<div class="row">
  {% for item in loop %}
    {% include '_item' with item %}
    {% include '_end_row' %}
  {% endfor %}
</div>