I currently have an events collection like so in my _config.yml:

future:true

collections:
  events:
    output: true
    permalink: /:collection/:name

I am trying to only show the next 4 future posts. However, I want to change the class of it's container based on if there is 0 or 1, 2 ,3 or 4 posts that are going to show up.

{% assign curDate = site.time | date: '%s' %}
{% for event in site.events %}
    {% assign postStartDate = event.date | date: '%s' %}
    {% if postStartDate >= curDate %}
         <div class="SOMETHING">{{event.title}}</div>
    {% endif %}
{% endfor %}

So if there is 1 post that has postStartDate >= curDate I want to use class .full, 2 posts: .one-half, 3 posts: .one-third, 4 posts: .one-quarter.

Does anyone now how I can count and determine this?

Try this

    {% assign curDate = site.time | date: '%s' %}
    {% assign post_count = site.events | size %}
    {% if post_count == 1 %}
    {% assign css_class = "full" %}
    {% elsif post_count == 2 %}
    {% assign css_class = "one-half" %}
    {% elsif post_count == 3 %}
    {% assign css_class = "one-third" %}
    {% else %}
    {% assign css_class = "one-quarter" %}
    {% endif %}

    {% for event in site.events %}
        {% assign postStartDate = event.date | date: '%s' %}
         {% if postStartDate > curDate %}
    <div class='{{css_class}}'>{{event.title}}</div>
        {% endif %}
    {% endfor %}