This is the error you get (emphasis mine):
The ORDER BY clause is invalid in
views, inline functions, derived
tables, subqueries, and common table
expressions, unless TOP or FOR XML is
So, how can you avoid the error? By specifying TOP, would be one possibility, I guess.
SELECT ( SELECT TOP 100 PERCENT COUNT(1) FROM Seanslar WHERE MONTH(tarihi) = 4 GROUP BY refKlinik_id ORDER BY refKlinik_id ) as dorduncuay