Solution: The after-tax income of (1) 4,000 yuan is 4,000-(4,000-800) ×14% = 3,552 yuan, from which it can be concluded that Uncle Zhang's pre-tax income should be higher than that of 800 yuan but not higher than 4,000 yuan.

Let uncle Zhang's pre-tax income be x yuan,

From the meaning of the question: x-(x-800)× 14%=3380,

Solution: x=3800 yuan.

(2) If you buy four boxes of "Buxue" brand tonic, you can get a bottle of "Lu" brand cosmetics, which is equivalent to 8.6% discount.

Therefore, you should first buy four boxes of "Buxue" brand supplements in store B, and then buy two bottles of "Lu" brand cosmetics in store A,

* * * The amount of money needed is: 4× 300+2× 200× 90% =1200+360 =1560 yuan.

A: Uncle Zhang's pre-tax expenses are 3,800 yuan. He should first buy four boxes of "Buxue" brand tonics in store B, and then buy two bottles of "Lu" brand cosmetics in store A, which needs the least money, * * * needs 1.560 yuan.