MySQL Question: ORDER BY /// DATEDIFF
In this case I would use a where clause to further qualify the data you are getting back in this case something like:

SELECT *, DATEDIFF(dateCompleted,dateRequested) AS Time
FROM ticket
WHERE ((dateCompleted IS NOT NULL) AND (dateRequested IS NOT NULL)) 
ORDER BY Time;


Since you said you are also trying to return only 4 rows you should use LIMIT 4 in your query, however I'm not sure if that would go before or after your ORDER BY.

If you have tried some things, let us know what you have already tried and what the errors are, this can help prevent people from reinventing the wheel and get your problem fixed faster.