Solution: according to the meaning of the question, set up x boxes that can produce boxed moon cakes at most:

2×0.05+4×0.02 = 4500

0. 1x+0.08x=4500

0. 18x=4500

x=25000

Flour for big moon cakes: 2x*0.05=2*25000*0.05=2500 (kg)

Flour for small moon cakes: 4x*0.02=4*25000*0.02=2000 (kg)

Answer: 2500kg flour is used for big moon cakes and 2000kg flour for small moon cakes.