4log2(u)-3 >= log²2(u)

log²2(u)-4log2(u)+3 <= 0

log2(u) = t

t²-4t+3 <= 0

t²-t-3t+3 <= 0

t(t-1)-3(t-1) <= 0

(t-1)(t-3) <= 0

+. -. +

------[1]------[3]------

1 <= t <= 3

1 <= log2(u) <= 3

2¹ <= 2^log2(u) <= 2³

2 <= u <= 8

u€[2; 8]