navbar not rendering in browser using ng-bootstrap & angular 4

As there is no navigation component you'd need to use a combination of the collapse functionality ng-bootstrap's ngbCollapse component as well as the dropdown functionality from the NgbDropdown component.

You'll need to bind an instance of ngbCollapse to the div.collapse.navbar-collapse and a boolean property on your component class. ng-bootstrap doesn't have any use for data-* so you can remove attributes such as data-toggle="collapse".

You control the open/close state of the collapse menu via a property on your component class that gets toggled true/false through something like a click event. In this example is toggle via a (click) event handler on the button.navbag-toggler executing a method on the component toggleMenu() which simply inverts the value of boolean property isCollapsed via the ! operator.

For the menu item dropdown menu, you'd use the NgbDropdown component. You'd apply attributes ngbDropdown and ngbDropdownToggle to the container element and toggle element respectively.

<div class="nav-item dropdown" ngbDropdown>
  <a class="nav-link dropdown-toggle" id="navbarDropdownMenuLink" ngbDropdownToggle>
    Dropdown
  </a>
  <div class="dropdown-menu" aria-labelledby="dropdownBasic1">
    <button class="dropdown-item">Action - 1</button>
    <button class="dropdown-item">Another Action</button>
    <button class="dropdown-item">Something else is here</button>
  </div>
</div>

In terms of styling for Bootstrap 4 navbar, you'll need to use the following classes:

Navbars require a wrapping .navbar with .navbar-toggleable-* for responsive collapsing and color scheme classes.

Also elements such as button.navbar-toggle are now button.navbar-toggler with an "r" in Bootstrap 4. You use classes navbar-inverse and bg-inverse for the standard inverse navbar.

HTML:

<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
  <button class="navbar-toggler navbar-toggler-right" type="button" aria-controls="appNavigation" [attr.aria-expanded]="!isCollapsed" aria-label="Toggle navigation" (click)="toggleMenu()">
    <span class="navbar-toggler-icon"></span>
  </button>
  <a class="navbar-brand" href="#">My App</a>
  <div class="collapse navbar-collapse" id="appNavigation" [ngbCollapse]="isCollapsed">
    <div class="navbar-nav mr-auto">
      <a class="nav-item nav-link" routerLink="" routerLinkActive="active">Home</a>
      <a class="nav-item nav-link" routerLink="/about" routerLinkActive="active">About</a>
      <div class="nav-item dropdown" ngbDropdown>
        <a class="nav-link dropdown-toggle" id="navbarDropdownMenuLink" ngbDropdownToggle>
            Dropdown
        </a>
        <div class="dropdown-menu" aria-labelledby="dropdownBasic1">
          <button class="dropdown-item">Action - 1</button>
          <button class="dropdown-item">Another Action</button>
          <button class="dropdown-item">Something else is here</button>
        </div>
      </div>
    </div>
  </div>
</nav>

TS:

export class NavigationComponent {
  isCollapsed = true;

  constructor() {}

  toggleMenu() {
    this.isCollapsed = !this.isCollapsed;
  }
}

Here is a plunker demonstrating the functionality and styling in action.