Let's put it into perspective:
(We're assuming all upgrades are 8 and that the hero is paralyzed/trapped/cannot evade these attacks)
Mech L4: 4000 damage with 8 upgrades (according to you, so I'm not doing any calculations)
Mech L3: (70 + 7*8) * 8 Tanks = 1008 Damage (explosive)
Mutant L3: (50 + 4*8) * 8 Lurkers * 2 Attacks = 1312 damage
Mutant L4: (150 + 12*8) * 12 attacks = 2952 damage (I'm assuming he can get off 12 attacks with it)
Volt L4: (80 + 8*8) * 15 Attacks = 2160 damage (I'm assuming he can get off 15 attacks with it)
(Am I the only one disappointed by Volt's L4 these days?)
Light Mage L1: (20 + 3*8) * 9 Templar (including LM himself here) * 3 Attacks = 1188 damage
Light Mage L2: (110 + 10*8) * 4 Reavers = 760 damage (lol stronger than L1)
Light Mage L4: (110 + 10*8) * 3 Reavers * 5 Waves = 2850 damage
Archer L1: (23 + 4*8) * 4 Mutalisks * 2 Attacks = 440 damage
Archer L4: (23 + 4*8) * 36 Mutalisks * 3 Attacks (probably more, even more damage if the Archer manual's before all the Mutalisks are made) = 5940 <-- !
Assassin's L4: (1800 + 8*20) = 1960 damage
Okay, that was more for me than for you.
I can see how the damage is a little high for Mech's L4. (and on Archer's L4 too) It's a bit tricky to balance a damage L4, since there really weren't any straight up damage L4's before, aside from Archer's. Considering it hasn't really even been playtested, I think you're the last person who should criticize me for not getting it right on the first try.
I want to test it more... if things don't work out I'll probably scale it back to 3 each of the Gols/Vults/Tanks instead of 4.