flex布局一：左對齊，且間距等分

ul {
    width: 400px;
    height: 250px;
    border: 1px solid green;
    margin: 0;
    padding: 0;
    list-style: none;
    /* flex */
    display: flex;
    flex-wrap: wrap;
}

li {
    width: 100px;
    height: 100px;
    border: 1px solid red;
    margin-left: calc((400px - 3*25%)/4);
}   

<ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
</ul>