How to display yahoo publisher ads after every 4 links

[slick33]

Is it possible to display yahoo publisher ads after every 4 links on home page? It should be placed in link_summary.tpl but what changes to be made to the code to display correctly.

Thanks,
SL

[cmstheme]

In link_summary.tpl:

PHP Code:

{if $link_shakebox_index eq '3'} {* display after the fourth link  *}
Your YPN Ad Code Hre
{/if}