(1) From the meaning of the question: y = 3000-4x-2 (1000-x) =1000-2x;

(2)∵x≥ 100,y≥0,∴ 100≤x≤500；

(3) From the meaning of the question: y= 1000-2x=0,

Solution: x=500,

A: Xiao Li wants to buy 500 Jin of oranges and 500 Jin of apples;

(4) From the meaning of the question: 1000-2x= 100.

Solution: x = 450.

Xiao Li should buy 450 kilograms of oranges and 550 kilograms of apples.