The GunGameMod server now has an automatic z_difficulty setting.
1-4 Players -> Normal
5-7 Players -> Advanced
8-10 Players -> Expert
You can vote upwards.
So you can play Expert with 4 players too.
Down votes are blocked.
e.g. 6 players cannot play Normal
Difficulty is only increased automatically, not decreased.
So Expert stays when e.g. 2 players disconnect from previous 9 players.
But the remaining 7 players can then vote for Advanced if they want to.
I hope I made it clear.
And I hope that it is working.
If it works, I may install it to all other servers too (should work for L4D also with adjusted Player counts because 7 is the maximum there at the moment)