Bootstrap 4 Dropdown not working

The btn-group class is no longer used for dropdowns in Bootstrap 4. Use the dropdown class for the parent:

<div class="dropdown">
  <button type="button" class="btn btn-danger dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Action
  </button>
  <div class="dropdown-menu">
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
    <a class="dropdown-item" href="#">Something else here</a>
    <div class="dropdown-divider"></div>
    <a class="dropdown-item" href="#">Separated link</a>
  </div>
</div>

Experienced the same issue with Alpha 4.0.6.

I've copied the .dropdown styles fra bootstrap.css 3.3.7 into dropdowns.less and it worked.

.dropup,
.dropdown {
  position: relative;
}

.dropdown-toggle::after {
  display: inline-block;
  width: 0;
  height: 0;
  margin-left: 0.3em;
  vertical-align: middle;
  content: "";
  border-top: 0.3em solid;
  border-right: 0.3em solid transparent;
  border-left: 0.3em solid transparent;
}

.dropdown-toggle:focus {
  outline: 0;
}

.dropup .dropdown-toggle::after {
  border-top: 0;
  border-bottom: 0.3em solid;
}

.dropdown-menu {
  position: absolute;
  top: 100%;
  left: 0;
  z-index: 1000;
  display: none;
  float: left;
  min-width: 10rem;
  padding: 0.5rem 0;
  margin: 0.125rem 0 0;
  font-size: 1rem;
  color: #292b2c;
  text-align: left;
  list-style: none;
  background-color: #fff;
  -webkit-background-clip: padding-box;
          background-clip: padding-box;
  border: 1px solid rgba(0, 0, 0, 0.15);
  border-radius: 0.25rem;
}

.dropdown-divider {
  height: 1px;
  margin: 0.5rem 0;
  overflow: hidden;
  background-color: #eceeef;
}

.dropdown-item {
  display: block;
  width: 100%;
  padding: 3px 1.5rem;
  clear: both;
  font-weight: normal;
  color: #292b2c;
  text-align: inherit;
  white-space: nowrap;
  background: none;
  border: 0;
}

.dropdown-item:focus, .dropdown-item:hover {
  color: #1d1e1f;
  text-decoration: none;
  background-color: #f7f7f9;
}

.dropdown-item.active, .dropdown-item:active {
  color: #fff;
  text-decoration: none;
  background-color: #0275d8;
}

.dropdown-item.disabled, .dropdown-item:disabled {
  color: #636c72;
  cursor: not-allowed;
  background-color: transparent;
}

.show > .dropdown-menu {
  display: block;
}

.show > a {
  outline: 0;
}

.dropdown-menu-right {
  right: 0;
  left: auto;
}

.dropdown-menu-left {
  right: auto;
  left: 0;
}

.dropdown-header {
  display: block;
  padding: 0.5rem 1.5rem;
  margin-bottom: 0;
  font-size: 0.875rem;
  color: #636c72;
  white-space: nowrap;
}

.dropdown-backdrop {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index: 990;
}

.dropup .dropdown-menu {
  top: auto;
  bottom: 100%;
  margin-bottom: 0.125rem;
}

Not a valid workaorund but it works :-)

Add this to your jQuery code

$('.dropdown').click(function(){
  $('.dropdown-menu').toggleClass('show');
});