How to limit length of inner divs inside another div using jquery

How to limit length of inner divs inside another div using jquery

like
<div class="a">
<div class="a1"></div>
<div class="a2"></div>
<div class="a3"></div>
<div class="a4"></div>
<div class="a5"></div>
<div class="a6"></div>
<div class="a7"></div>
<div class="a8"></div>
</div>
i need to limit the length of div 'a' to 4 using jquery. Thanks in advance