how to float div items fill in space div parent?

<div class="parent">
    <div class="child">1</div>
    <div class="child">2</div>
    <div class="child">3</div>
    <div class="child-wide">4</div>
    <div class="child">5</div>
    <div class="child">6</div>
    <div class="child">7</div>
</div>

.parent { 
width:408px; 
display:inline-block; 
background-color:orange;
color:white;
}
.parent div {
float:left;
height:100px;
background-color:green;
border:1px solid white;
font-size:36px;
text-align:center;
}
.child { width:100px;}
.child-wide { width:202px;}