Where am I wrong precisely please ? 1) Let Avast! automatically connect every 4 hours is not possible because 2 machines doing so for only 8 mn every 4 hours leads to consuming a 50h monthly subscription;
That is where you go wrong. Avast does not connect every 4 hours. It only checks when a connection is made. It does NOT make the connection itself. EG: If you do not connect for over a month, you won't have any (extra) costs.

Avast checks every 4 hours after the connection is made, but only if the connection is open for 4 hours or longer. Again, Avast does NOT connect by itself.

For the other thing. What os is on the server?