Twitter Bootstrap: make a 2x2 grid - css

Question

I have the following:
<div class="container-fluid">
<div class="row-fluid">
<div class="span3"></div>
<div class="span3"></div>
<div class="span3"></div>
<div class="span3"></div>
</div>
</div>
By default, this div.span* spans the entire width of the screen, like this:
[x][x][x][x]
At a certain screen width, I want this to appear in a 2x2 grid, like this:
[x][x]
[x][x]
How do I do this?

Answer 1

Sorry about my earlier attempts, I did not fully understand your question:
The thing which you are trying with bootstrap is not really possible unless you go for your own #media selectors. There is a library called Neat. I think this is the example you are looking for.
EARLIER ATTEMPTS:
Try this, from here:
<div class="container-fluid">
<div class="row-fluid">
<div class="span6">
<div class="row-fluid">
<div class="span6">A</div>
<div class="span6">B</div>
</div>
<div class="row-fluid">
<div class="span6">C</div>
<div class="span6">D</div>
</div>
</div>
</div>
</div>
This should give you the following result:
[A][B]
[C][D]
Well that's a lot of divs. Not really sure if this can be made lighter.

Answer 2

The original question appears to be for an older edition of bootstrap.
Here's what solves the issue neatly in Bootstrap 3 markup. The key element is the clearfix div that affects xs and sm viewports [typical use case]. (sm not included in example below).
<div class="row">
<div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
<div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
<!-- Add the extra clearfix for only the required viewport -->
<div class="clearfix visible-xs-block"></div>
<div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
<div class="col-xs-6 col-sm-3">.col-xs-6 .col-sm-3</div>
</div>
via getbootstrap.com

Answer 3

Here are 2 options that are responsive without the need for media queries. Resize the windows to see how they react.
CSS Columns:
http://jsfiddle.net/88t4L/
.row-fluid {
columns: 2 8em;
}
Here, the columns must be at least 8em wide, but if there's room for all of them to appear in a row, it will do so.
http://caniuse.com/#search=columns
CSS Flexbox:
http://jsfiddle.net/88t4L/1/
.row-fluid {
display: flex;
flex-flow: row wrap;
}
.row-fluid .span3 {
flex: 1 0 8em; /* grow equally, don't shrink, preferred width of 8em */
}
http://caniuse.com/#search=flexbox