Electric field at distance r ⇒E= λ/2πrϵ0
Flux through Gaussian cylinder
ϕ=E.2πrL
ϕ= λL/ϵ0
According to question, If we set up 3 or more identical cube around it, to cover its area then we get flux equal to that of cylindrical Gaussian surface i.e. λL/ϵ0
But as we are covered with only 1/4 of area thus
Flux would be = λL/4ϵ0