div#level_1 {
 z-index: 10;
 display: inline;
 background-color: #333333;
}

div#level_2 {
 z-index: 11;
 display: none;
 background-color: #666666; 
}

div#level_3 {
 z-index: 12;
 display: none;
 background-color: #999999;
}

div#level_4 {
 z-index: 13;
 display: none;
 background-color: #cccccc; 
}