--End of last quarter
DATEADD(qq,DATEDIFF(qq,0,GETDATE()),-1)
-- start of four quarters ago
DATEADD(qq,DATEDIFF(qq,0,GETDATE())-4,0);
-- to use this in a WHERE clause, I would use a slightly different approach
-- as shown in red below:
WHERE
YourDateCol >= DATEADD(qq,DATEDIFF(qq,0,GETDATE())-4,0)
AND YourDateCol < DATEADD(qq,DATEDIFF(qq,0,GETDATE()),0)