The biggest problem I see with the IV turret so far is the extremely limiting Knowledge cost factor.
I used them in defense, and they tear through starships and smaller ships with fairly-accurate regard to their costs, except I don't think I can justify spending 6000 K on one turret in a non-cheat game.
Really, the whole Heavy-Turret line is very expensive Knowledge-wise. 15,000 Knowledge in total, that's 8 planets worth of knowledge dedicated to an almost purely defensive structure. In an 80-planet game, that's a little less than 10% of my total knowledge.
For 15,000 Knowledge, I could unlock MK II and MK III ships in two different lines, along with MK II Scouts (560 ships total on non-bonus ships)
For 15,000 Knowledge, I could unlock MK I and MK II Raid and Dreadnought Starships, along with the MK III Raid Starship (19 Mobile, high-powered Starships)
For 14,500 Knowledge, I could unlock MK I - MK III Missile Turrets, MK II + MK III MLRS Turrets, and MK I + MK II Laser Turrets (1190 Stationary, low-powered static defenses)
For 15,000 Knowledge, I could unlock MK I - MK IV Heavy turrets (25 Stationary, high-powered static defenses)
The first three allow a great amount of flexibility in deployment, while maintaining a very large amount of raw power.
The last allows an extremely large amount of raw power, but the flexibility is lost because we only get 25 of them.
After a certain point, firepower no longer has the same amount of importance as before. Knowledge is the only finite resource in the game, and after a certain point, REQUIRES the destruction of enemy sectors to effectively increase. Knowledge is a very important resource, and I really don't think the heavy turrets are THAT justified.
But in any game with "give me k", they'll be among the first turrets I deploy. They rock when I "accidentally" nuke a planet with 5+ Enemy Starships.