为什么css3：nth-​​child（4n）计数不正确

Question

我的目标是每一行有四列我使用:nth-child但不能正常工作，因为我预计只能在第一行工作，第二行边界div在第三列。< / p>

HTML

    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>

    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>
    <div class="workplace">some text here</div>

    <div class="workplace">some text here</div>

css

 .workplace:nth-child(4n){
    border:1px solid red;
    }

我的问题是：为什么会发生这种情况是因为其他风格的冲突？