令f(x)满足：f(1)=1/2；f(2)=1/4；f(3)=1/4；得f(x)。从而得f(4)。

方法之一：令f(x)是一个二次函数。
f(x) = a*x^2 + b*x + c

用系数待定法求a，b，c

f(1) = a + b + c = 1/2 ----------------------(1)

f(2) = 4a + 2b + c = 1/4 ---------------------(2)

f(3) = 9a + 3b + c = 1/4 ---------------------(3)

(3)-(2)
5a + b = 0 ---------------------------(4)

(2)-(1)
3a + b = -1/4 ------------------------(5)

(4)-(5)
2a = 1/4
a = 1/8

将a值代入(5)得
b = -5/8

将a，b值代入(1)得
c = 1/2 - 1/8 + 5/8 = 1

将a，b，c 值代入f(x)得
f(x)= 1/8*x^2 - 5/8*x + 1

令x = 4

f(4)= 1/8*16 - 5/8*4 + 1 = 2 - 5/2 + 1 = 1/2