设 y = kx + b/x
x=1时y=4,y = k + b = 4
x=2时y=5 y = 2k + b/2 = 5
所以,k = 2 b = 2
所以,y = 2x + 2/x
所以,当x=4时,y = 2×4 + 2/4 = 8.5