  The 4/5-law of turbulence, which characterizes the energy cascade from large
to small-sized eddies at high Reynolds numbers in classical fluids, is verified
experimentally in a superfluid 4He wind tunnel, operated down to 1.56 K and up
to R_lambda ~ 1640. The result is corroborated by high-resolution simulations
of Landau-Tisza's two-fluid model down to 1.15 K, corresponding to a residual
normal fluid concentration below 3 % but with a lower Reynolds number of order
R_lambda ~ 100. Although the K\'arm\'an-Howarth equation (including a viscous
term) is not valid \emph{a priori} in a superfluid, it is found that it
provides an empirical description of the deviation from the ideal 4/5-law at
small scales and allows us to identify an effective viscosity for the
superfluid, whose value matches the kinematic viscosity of the normal fluid
regardless of its concentration.
