Ziwei Garden lacks management.

(1) If the width of the garden is x meters, the length of the garden is 40-2x;

(2) The garden wall is x meters wide and the garden area is 200 square meters.

x(40-2x)=200，

The solution is x= 10,

When the width of the garden is10m, the garden area can reach 200m2.

(3) The garden fence is x meters long and the garden area is 250 square meters.

x(40-2x)=250，

That is 2x2-40x+250=0,

△= B2-4ac = 1600-4×2×250 =-400 < 0，

Therefore, the garden area cannot reach 250m2. ..

(4) The garden fence is x meters long and the garden area is y square meters.

According to the meaning of the question, y=x(40-2x)=-2x2+40x,

When x=-b2a=-402× (? 2)= 10, the garden area is the largest;

That is, the width of the garden is 10m and the length is 20 m.