SOURCE

<div class="row">
    <div class="col">
        <div class="content">1</div>
    </div>
    <div class="col">
        <div class="content">2</div>
    </div>
    <div class="col">
        <div class="content">3</div>
    </div>
      <div class="col">
        <div class="content">3</div>
    </div>
</div>

body {
    margin: 0;
    --col-width:25%;
}

* {
    box-sizing: border-box;
}

.row {
    margin: 0 -12px;
    display: flex;
    flex-wrap: wrap;
}

.col {
    padding: 0 12px;
    width: var(--col-width);
    flex: 0 0 var(--col-width);
}

.content {
    background: #ddd;
    width: 100%;
    text-align: center;
}