CSS3 Challenge #1: Flexbox

Since I create videos only to help beginners, this is a very basic challenge on flexbox. Please try complete on your own first .
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="style.css">
    <title>Document</title>
</head>
<body>
    
    <div class="container">
        <div class="inner">
                <div>1</div>
                <div>2</div>
                <div>3</div>
        </div>        
        <div>4</div>
    </div>

</body>
</html>

 

.container{
    width: 500px;
    border: 1px solid black;
    display: flex;
    justify-content: space-between;
}

.inner {
    display: flex;    
}

.container div+div, .inner div {
    height: 100px;
    border: 1px solid green;
    text-align: center;
    box-sizing: border-box;
    padding: 35px;
}