Ich würde Baldurs SQL noch etwas modifizieren:

Code:
With x as (Select c.datum
             From Calendar c join Bestand b 
                  on b.Datum between     c.Datum and c.datum + 3 Days  
                                     and TrueFalse = 1    
             Group by c.Datum
             Having Count(c.Datum) >= 4)
Select b.* 
from x join Bestand b on x.Datum = b.Datum and TrueFalse = 1
Birgitta