Risolto!
<%
StrSql="SELECT * FROM tbl_prodotti WHERE usato='SI' ORDER BY idprodotto DESC"
RS.Open StrSql, CN, 1
IF NOT RS.EOF THEN
DO While NOT RS.EOF
I=I+1
%>
<% IF I=4 THEN %>
</ul>
<ul class="product-grid">
<%
END IF
%>