<script type="text/javascript">

        $(function(){

             var userAgent = navigator.userAgent;

             var index = userAgent.indexOf("Android");

             if(index >= 0){

               var androidVersion = parseFloat(userAgent.slice(index+8));

               alert(androidVersion);//系统版本 例4.2

               if(androidVersion<4.2){

                // 版本小于4.2的事情

                //.....

               }

        });

</script>