Okay, my guild unlocked lvl 4 this week and here's my feedback.... IT SUCKS !!!
I'll elaborate:
1) Very very costly to unlock lvl 4
2) I know Inno said that lvl 4 is only for the "hardcore" players, but c'mon! It's like sending someone to a gunfight equipped only with a knife and telling them "yeah, it's gonna be hard". I'm in HMA on beta, how high is supposed to be my attack boost? #4 encounter on lvl 4 is a full 2-wave army at 25% boost...I didn't even try to fight it. I spent tons of goods to negotiate first 4 and I got 2 fps, 20 goods, 80 medals and a Gate of the Sun God. A gate of the Sun God yay ... I already got 3 of those from relics ..*end sarcasm mode*
3) Gate of the Sun God/Face of the Ancient is imba in HMA. It's 2x2 and gives 479 happiness as opposed to the the Doctor's (HMA cultural building) which is 3x3, requires a road and gives 470 happiness. (I know you're gonna nerf it now).
I'm not aware of the stats of those in AF, but if it's the same as the flower arbor, I see a very bad balancing here!!! Following that logic those 2 should give at least 5000 happiness in AF, but something tells me they don't...
4) With all that said, why would any guild spend tons of goods for something which only a few members will be able to play, would cost them significant amount of goods/troops (or both) to gain prizes not matching the effort like a flower arbor equivalent, a couple of FPs, a nerfed SoK, possibly a few diamonds and TONS of EXTRA ATTEMPTS ? To be honest, I never would...
Summary:
GOOD idea but VERY BAD performance