Edit in JSFiddle

<ul class="dropdown">
    <li>
        <a href="#">Item1</a>
        <ul class="nest">
            <li>Item1-1</li>
            <li>Item1-2</li>
            <li>Item1-3</li>
        </ul>
    </li>
    <li>
        <a href="#">Item2</a>
        <ul class="nest">
            <li>Item2-1</li>
            <li>Item2-2</li>
            <li>Item2-3</li>
            <li>Item2-4</li>
            <li>Item2-5</li>
        </ul>
    </li>
    <li>
        <a href="#">Item3</a>
        <ul class="nest">
            <li>Item3-1</li>
            <li>Item3-2</li>
            <li>Item3-3</li>
            <li>Item3-4</li>
        </ul>    
    </li>
    <li>
        <a href="#">Item4</a>
        <ul class="nest">
            <li>Item4-1</li>
            <li>Item4-2</li>
        </ul>    
    </li>
</ul>

body {
    overflow: hidden;
    font-family:'Lucida Grande','Hiragino Kaku Gothic ProN',Meiryo, sans-serif;
    color: #343434;
    background: #454545;
}

a {
    color: inherit;
    text-decoration: none;
}

ul {
    list-style: none;
}

.dropdown {
    padding: 0;
    background-color: #73BF67;
    border-top-left-radius: 5px;
    border-top-right-radius: 5px;
    *zoom: 1;
}

.dropdown:before, .dropdown:after {
    content: "";
    display: table;
}

.dropdown:after {
    clear: both;
}
    
.dropdown > li {
    width: 25%;
    float: left;
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    height: 2em;
    line-height: 2em;
    position: relative;
}
        
.dropdown > li a {
    width: 100%;
    text-align: center;
    display: block;
}

.dropdown > li:not(:last-child) a {
    border-right: 1px solid rgba(60, 60 ,60, .2);
}

.dropdown > li:hover .nest > li {
    opacity: 1;
    background-color: #D1F1CC;
}

.nest {
    font-size: .8em;
    height: auto;
    padding: 0;
    width: 100%;
    position: absolute;
    z-index: 1;
    transition: height .2s linear 0s;
    -webkit-transition: height .2s linear 0s;
    -moz-transition: height .2s linear 0s;
    overflow: hidden;
 }
    
.nest > li {
    width: 100%;
    border-bottom: 1px solid rgba(60, 60, 60, .2);
    padding-left: 1em;
    transition: color .2s linear 0s;
    -webkit-transition: color .2s linear 0s;
    -moz-transition: color .2s linear 0s;
    cursor: pointer;
    opacity: 0;
    background-color: transparent;
}

.nest > li:before {
    content: "くコ:彡　";
}

.nest > li:first-child {
    transition: opacity,background-color .2s linear 0s;
    -webkit-transition: opacity,background-color .2s linear 0s;
    -moz-transition: opacity,background-color .2s linear 0s;
}

.nest > li:first-child:after {
    content: "";
    position: absolute;
    top: 0;
    left: 6.6em;
    border-left: 1em solid transparent;
    border-right: 1em solid transparent;
    border-top: 1em solid #73BF67;
}

.nest > li:nth-child(2) {
    transition: opacity,background-color .2s linear 0.05s;
    -webkit-transition:  opacity,background-color .2s linear 0.05s;
    -moz-transition: opacity,background-color .2s linear 0.05s;
}

.nest > li:nth-child(3) {
    transition:  opacity,background-color .2s linear 0.1s;
    -webkit-transition: opacity,background-color .2s linear 0.1s;
    -moz-transition: opacity,background-color .2s linear 0.1s;
}

.nest > li:nth-child(4) {
    transition: opacity,background-color .2s linear 0.15s;
    -webkit-transition: opacity,background-color .2s linear 0.15s;
    -moz-transition: opacity,background-color .2s linear 0.15s;
}

.nest > li:nth-child(5) {
    transition: opacity,background-color .2s linear 0.2s;
    -webkit-transition: opacity,background-color .2s linear 0.2s;
    -moz-transition: opacity,background-color .2s linear 0.2s;
}


.nest > li:hover {
    color: #73BF67;
}

.nest > li:first-child {
    padding-top: .8em;
}