.ornek2{
width:100px;
height:40px;
background:#096;
color:#FFF;
}
.ornek2:hover{
background-color:#036;
-o-transition:background-color 4s;
-webkit-transition: background-color 4s ;
-moz-transition: background-color 4s ;
}
<div class="ornek2">4 saniyede arka plan rengi değişir</div>