Bootstrap 4 navbar with 2 rows

You can use the flex-column flexbox utility class to stack the 2 navs vertically next to the icon. This sets flex-direction: column on the navbar-collapse div so that it's child elements stack vertically.

<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
    <div class="container">
        <button class="navbar-toggler navbar-toggler-right align-self-center mt-3" type="button" data-toggle="collapse" data-target="#navbarCollapse">
            <span class="navbar-toggler-icon"></span>
        </button>
        <h1 class="py-2 ml-lg-2 mx-3"><a href="#"><i class="fa fa-envelope-o fa-lg mt-2" aria-hidden="true"></i></a></h1>
        <div class="collapse navbar-collapse flex-column ml-lg-0 ml-3" id="navbarCollapse">
            <ul class="navbar-nav">
                <li class="nav-item active">
                    <a class="nav-link" href="#">Home</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Product</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Shop</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">About</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Events</a>
                </li>
            </ul>
            <ul class="navbar-nav flex-row mb-2">
                <li class="nav-item">
                    <a class="nav-link py-1 pr-3" href="#"><i class="fa fa-facebook"></i></a>
                </li>
                <li class="nav-item">
                    <a class="nav-link py-1 pr-3" href="#"><i class="fa fa-instagram"></i></a>
                </li>
                <li class="nav-item">
                    <a class="nav-link py-1 pr-3" href="#"><i class="fa fa-twitter"></i></a>
                </li>
            </ul>
        </div>
    </div>
</nav>

Demo - Bootstrap 4 navbar with 2 rows

Here's another variation with the 2 rows and right align search form:

<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
    <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarCollapse">
        <span class="navbar-toggler-icon"></span>
    </button>
    <a class="navbar-brand my-auto" href="#">Brand</a>
    <div class="collapse navbar-collapse flex-md-column" id="navbarCollapse">
        <ul class="navbar-nav ml-auto small">
            <li class="nav-item active">
                <a class="nav-link" href="#">Shop</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Products</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Team</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">About</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Events</a>
            </li>
        </ul>
        <form class="form-inline ml-auto">
            ...
        </form>
    </div>
</nav>

Demo - Navbar with 2 rows on right

Update for Bootstrap 4.0.0:
https://www.codeply.com/go/05hEHoiUvv

Here is an alternate scenario with 2 navbars with one mobile toggler

Bootstrap 4.0 - Two NavBars that collapse into one toggle
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Bootstrap 4 Multiple fixed-top navbars

The "variation with the 2 row aligned right" did not right-align the rows in Safari for me. I put the unordered lists in a new div with the flex-column and ml-auto classes.

<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
    <div class="container">
      <button class="navbar-toggler navbar-toggler-right align-self-center mt-3" type="button" data-toggle="collapse" data-target="#navbarCollapse"> <span class="navbar-toggler-icon"></span> </button>
      <h1 class="py-2 ml-lg-2 mx-3"><a href="#"><i class="fa fa-envelope-open-o fa-lg" aria-hidden="true"></i></a></h1>
      <div class="collapse navbar-collapse ml-lg-0 ml-3" id="navbarCollapse">
        <div class="flex-column ml-auto">
          <ul class="navbar-nav">
            <li class="nav-item active"> <a class="nav-link" href="#">Home</a> </li>
            <li class="nav-item"> <a class="nav-link" href="#">Product</a> </li>
            <li class="nav-item"> <a class="nav-link" href="#">Shop</a> </li>
            <li class="nav-item"> <a class="nav-link" href="#">About</a> </li>
            <li class="nav-item"> <a class="nav-link" href="#">Events</a> </li>
          </ul>
          <ul class="navbar-nav">
            <li class="nav-item"> <a class="nav-link pr-3" href="#"><i class="fa fa-facebook"></i></a> </li>
            <li class="nav-item"> <a class="nav-link pr-3" href="#"><i class="fa fa-instagram"></i></a> </li>
            <li class="nav-item"> <a class="nav-link pr-3" href="#"><i class="fa fa-twitter"></i></a> </li>
          </ul>
        </div>
      </div>
    </div>
  </nav>