Bootstrap4のnavbar-navをセンター寄せ：デモページ

<nav class="navbar navbar-expand-lg navbar-dark bg-dark">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbar1" aria-controls="navbar1" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse justify-content-center" id="navbar1">
    <div class="navbar-nav">
      <a class="nav-item nav-link" href="#">メニュー1</a>
      <a class="nav-item nav-link" href="#">メニュー2</a>
      <a class="nav-item nav-link" href="#">メニュー3</a>
      <a class="nav-item nav-link" href="#">メニュー4</a>
    </div>
  </div>
</nav>

@media (min-width: 768px) and (max-width: 991px) {
  .navbar-nav .nav-link{
    padding:0 1rem;
  }
}
@media (min-width: 992px) and (max-width: 1199px) {
  .navbar-nav .nav-link{
    padding:0 2rem;
  }
}
@media (min-width: 1200px) {
  .navbar-nav .nav-link{
    padding:0 3rem;
  }
}