You can achieve this by using the_excerpt filter. You can read further on the codex. Just paste this code in functions.php and set the value of $i = no of words from the end you want.
add_filter('the_excerpt','my_excerpt');
function my_excerpt(){
global $post;
$excerpt=get_the_excerpt();
$content = get_the_content(); //gets the whole content
$content =strip_tags($content) ; //strips html tags
$content = explode(" ", $content); //stores each word in an array
$size = count($content); //counts the length of array
$last=""; //initialize an empty sting
$i=4; //no of words from the last you want
while($i>0){
$last.= $content[$size-$i];
$last.=" ";
$i--;
}
return $excerpt."......".$last;
}
Now use the_excerpt();
to output the kind excerpt you want.
All the best and let me know if it worked or not.