<style>
    .flexbox {
        display: flex;
    }

    .box-example .box {
        width: 150px;
        height: 150px;
    }
</style>
<div class="flexbox box-example">
    <div class="box">
        <h3>Box Level 1</h3>
    </div>
    <div class="box bg-level-2">
        <h3>Box Level 2</h3>
    </div>
    <div class="box bg-level-3">
        <h3>Box Level 3</h3>
    </div>
    <div class="box bg-level-4">
        <h3>Box Level 4</h3>
    </div>
</div>