display: flex; で中央に寄せる方法 

<head>
    <link rel="stylesheet" href="sample.css">
</head>
<div class="items">
    <div class="item">Item01</div>
    <div class="item">Item02</div>
    <div class="item">Item03</div>
    <div class="item">Item04</div>
</div>

.items {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 1000px;
    height: 500px;
    gap: 10px;
    background-color: green;
}

.item {
    width: 150px;
    height: 300px;
    background-color: black;
    color: white;
    display: flex;
    justify-content: center;
    align-items: center;
}

sample