(2) The discounted price is:1.5a× 0.8 =1.2a;

(3) 1.2a-a=0.2a，

That is, the profit of each point reading machine is 0.2a yuan;

(4) According to the meaning of the question: 40× 0.2a+10× (0.5×1.5a-a) = 5.5a = 5.5× 800 = 4,400 (yuan).

Answer: The merchants made a profit of 4,400 yuan by selling this batch of point readers.