asdf.html.erb

<% categories_column_size = (categories.size / 4).ceil %>

<div class="row">  
  <% # the false on the next line removes the nils you used to protect with the if category %>
  <% categories.in_groups_of(categories_column_size, false) do |n_categories| %>
    <div class="large-3 columns">
      <% n_categories.each do |category| %>
        <div class="row">
          <div class="large-10 columns">
            <%= label_tag category.name, category.name, class: 'right inline' %>
          </div>
          <div class="large-2 columns">
            <%= check_box_tag "categories[#{category.id}]", "1", false, class: 'left inline' %>
          </div>
        </div>
      <% end %>
  </div>
</div>