输入组的非常简单的例子：
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<h3>Input Groups</h3>
<p>The .input-group class is a container to enhance an input by adding an icon, text or a button in front or behind it as a "help text".</p>
<p>The .input-group-addon class attaches an icon or help text next to the input field.</p>
<form>
<div class="input-group">
<span class="input-group-addon"><i class="glyphicon glyphicon-user"></i></span>
<input id="email" type="text" class="form-control" name="email" placeholder="Email">
</div>
<div class="input-group">
<span class="input-group-addon"><i class="glyphicon glyphicon-lock"></i></span>
<input id="password" type="password" class="form-control" name="password" placeholder="Password">
</div>
<br>
<div class="input-group">
<span class="input-group-addon">Text</span>
<input id="msg" type="text" class="form-control" name="msg" placeholder="Additional Info">
</div>
</form>
<br>
<p>It can also be used on the right side of the input:</p>
<form>
<div class="input-group">
<input id="email" type="text" class="form-control" name="email" placeholder="Email">
<span class="input-group-addon"><i class="glyphicon glyphicon-user"></i></span>
</div>
<div class="input-group">
<input id="password" type="password" class="form-control" name="password" placeholder="Password">
<span class="input-group-addon"><i class="glyphicon glyphicon-lock"></i></span>
</div>
</form>
</div>
</body>
</html>
工作正常。但是当我使用最新版本（bootstrap 4）时，设计已被打破。
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<h3>Input Groups</h3>
<p>The .input-group class is a container to enhance an input by adding an icon, text or a button in front or behind it as a "help text".</p>
<p>The .input-group-addon class attaches an icon or help text next to the input field.</p>
<form>
<div class="input-group">
<span class="input-group-addon"><i class="glyphicon glyphicon-user"></i></span>
<input id="email" type="text" class="form-control" name="email" placeholder="Email">
</div>
<div class="input-group">
<span class="input-group-addon"><i class="glyphicon glyphicon-lock"></i></span>
<input id="password" type="password" class="form-control" name="password" placeholder="Password">
</div>
<br>
<div class="input-group">
<span class="input-group-addon">Text</span>
<input id="msg" type="text" class="form-control" name="msg" placeholder="Additional Info">
</div>
</form>
<br>
<p>It can also be used on the right side of the input:</p>
<form>
<div class="input-group">
<input id="email" type="text" class="form-control" name="email" placeholder="Email">
<span class="input-group-addon"><i class="glyphicon glyphicon-user"></i></span>
</div>
<div class="input-group">
<input id="password" type="password" class="form-control" name="password" placeholder="Password">
<span class="input-group-addon"><i class="glyphicon glyphicon-lock"></i></span>
</div>
</form>
</div>
</body>
</html>
除了bootstrap版本之外，这两个示例是100％相同的。那么第4版的问题是什么？