Why Scan All 38K Rows When I Limit 4?

[casbboy]

I've been using EXPLAIN to better do my queries and spotted this:

EXPLAIN SELECT films.film_title, trailers.trailer_title 
FROM films,trailers 
WHERE trailers.film_id=films.film_id 
ORDER BY trailers.trailer_id DESC 
LIMIT 4 

The result I get is 48K rows must get scanned in the 'trailers' table. Why can't it just do the 4 and stop?

Thanks
Ryan