it shoud be

DECLARE @comparedate AS varchar(20)

SET @comparedate = convert(varchar(20), dateadd(D,-4, GETDATE()), 102)

declare @abfrage varchar(2050)

SET @abfrage='SELECT Anlagedatum, Artikel FROM S_artikel WHERE Firma = ''200'' AND convert(varchar(20),Anlagedatum,102) >=''' + @comparedate + ''' ORDER BY Anlagedatum'

if your anlagedatum is datetime type, if not then convert first to datetime

and convert to varchar in format you want (102, 104 ...).

be cautious when use Dynamic query from sql injection