These 4 items should fill the width:<br/>
<ul id="ul1">
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
</ul>
<br/>
These 5 items should fill the width:<br/>
<ul id="ul2">
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
</ul>
<br/>
And so on and so forth...

ul {
    border:1px dotted black;
    padding: 0;
    display:table;
    width:100%;
    box-sizing:border-box;
    -moz-box-sizing:border-box;
    -webkit-box-sizing:border-box;
}
li {
    background-color: red;
    display:table-cell;
}
li:nth-child(2n) {
    background-color: #0F0;
}

.li2Elem{width:50%}
.li3Elem{width:33.3%}
.li4Elem{width:25%}
.li5Elem{width:20%}