Code Run - Live Demo

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Wrapping of Flex Items inside Flex Container</title>
<style> 
.flex-container {
    width: 500px;
    min-height: 320px;
    margin: 0 auto;
    font-size: 32px;
    border: 5px solid black;
    /* Safari */
    display: -webkit-flex;
    -webkit-flex-wrap: wrap;
    /* Standard syntax */
    display: flex;
    flex-wrap: wrap;  
}
.flex-container div{
    padding: 10px;
    width: 130px;
}
.item1 {
    background: yellow;
}
.item2 {
    background: blue;
}
.item3 {
    background: purple;
}
.item4 {
    background: orange;
}
</style>
</head>
<body>
    <div class="flex-container">
        <div class="item1">1</div>
        <div class="item2">2</div>
        <div class="item3">3</div>
        <div class="item4">4</div>
    </div>
    <p><strong>Note:</strong> The other values for the align-items property like, "flex-start", "flex-end", "baseline" and "stretch" to understand how it aligns the items inside flex container.</p>
</body>
</html>