The relay of messages

1. Optimal strategy: A calls B $\Rightarrow$ A and B know $\{a,b\}$.
2. With 3 calls it is not possible for the four to accumulate complete information.
3. Response (4 spies): minimum 4 calls.

Back to the problem
Response (4 spies): minimum 4 calls.
Optimal strategy:

1. A calls B $\Rightarrow$ A and B know $\{a,b\}$.
2. C calls D $\Rightarrow$ C and D know $\{c,d\}$.
3. A calls C $\Rightarrow$ A and C know $\{a,b,c,d\}$.
4. B calls D $\Rightarrow$ B and D also know everything.

With 3 calls it is not possible for the four to accumulate complete information.
Generalization: for $n\ge4$, the minimum is:

$$ 2n-4. $$