最后，我管理了HTML和您的HTML. 与版本3相比，Bootstrap 4有很多更改.关于标记，您必须进行以下更改:

Finally, I managed my HTML and yours. There're a lot of changes in Bootstrap 4 in compare to version 3. Regarding your markup, you have to change:

1. 将"Navbar-inverse"改为"Navbar-dark"，并使用颜色"bg-dark".
2. 为按钮添加一些属性，例如"aria-controls"，"aria-expanded"，"aria-label"和"data-target"，以链接到另一个元素.
3. 可折叠元素的属性"id".
4. 对于列表元素(标签LI)，应添加class ="nav-item"
5. 对于链接之外的列表元素，请添加class ="nav-link".
6. 我建议在"definiton"列表中添加"mr-auto".

下面的所有更改.在此处进行了测试.

All changes below. Tested here.

<nav class="navbar navbar-expand-sm navbar-dark fixed-top bg-dark">
  <div class="container">
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    </button>
    <a class="navbar-brand" href="/">Application name</a>
    <div class="navbar-collapse collapse" id="navbarSupportedContent">
      <ul class="nav navbar-nav mr-auto">
        <li class="nav-item"><a href="/" class="nav-link">Home</a></li>
        <li class="nav-item"><a href="/Home/About" class="nav-link">About</a></li>
        <li class="nav-item"><a href="/Home/Contact" class="nav-link">Contact</a></li>
      </ul>

      <ul class="nav navbar-nav navbar-right mr-auto">
        <li class="nav-item"><a href="/Account/Register" id="registerLink" class="nav-link">Register</a></li>
        <li class="nav-item"><a href="/Account/Login" id="loginLink" class="nav-link">Log in</a></li>
      </ul>

    </div>
  </div>
</nav>