SCENARIO TWO Bombardment

[g1ul10]

I'm not sure on how to interpret the special rule:

The Soviet player may call for one randomly drawn aircraft
on any 4 turns. After the first aircraft is drawn, roll one die
on each subsequent call for an aircraft, on a 4-6 the arriving
aircraft is German.

What does it mean "on any 4 turns"? They can call it on turn 1 and then again on turn 5? Or on turn 2 and then again on turn 6? If they call it in turn 3, they can't call it anymore? And the aircraft, once "called", can try to attack in each turn or only in the turn it is called? I think the second, otherwise, it would be absurd not to call it right away in turn 1...