JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">Some content 1</div>
<div class="block">Some content 2</div>
<div class="block">Some content 3</div>
<div class="block">Some content 4</div>
<div class="block">Some content 5</div>
<div class="block">Some content 6</div>
<div class="block">Some content 7</div>
<div class="block">Some content 8</div>
<div class="block">Some content 9</div>
<div class="block">Some content 10</div>
<div class="block">Some content 11</div>
<div class="block">Some content 12</div>
<div class="block">Some content 13</div>
<div class="block">Some content 14</div>
<div class="block">Some content 15</div>
<div class="block">Some content 16</div>
<div class="block">Some content 17</div>
<div class="block">Some content 18</div>

CSS

* {
    -webkit-box-sizing: border-box;
       -moz-box-sizing: border-box;
            box-sizing: border-box;
}
body {
    width: 400px;
    background-color: lightgreen;
}
.block {
    float: left;
    width: 24%;
    min-height: 50px;
    margin: 0 1% 1% 0;
    padding: 4px;
    border: 1px solid #aaa;
    background-color: lightblue;
}
/* Improvements for last "row" http://slides.com/heydon/effortless-style#/45 and next slides */
.block:nth-child(4n+1):last-child {
    width: 99%;
}
.block:nth-child(4n+1):nth-last-child(2),
.block:nth-child(4n+2):last-child {
    width: 49%;
}
.block:nth-child(4n+1):nth-last-child(3),
.block:nth-child(4n+2):nth-last-child(2),
.block:nth-child(4n+3):last-child {
    width: 32%;
}

JavaScript

/* SO http://stackoverflow.com/questions/24225809/how-to-make-4-cells-in-each-line-by-only-css-html */