I agree that the pirates are too weak. To start off with, unless they are next to a player they stand *no* chance at all to be honest.
Even when they do come across a human player, that human player usually recognises quickly that they are pirates and counters accordingly (armed escorts, fleet movements rather than lone ships and so on)
Not sure how to make them stronger though. I suggest playtesting lots of things
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The Unofficial Space Empires 4 Mod Utility V2.0
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Download as a .ZIP <
HERE>
Download as a .RAR <
HERE>
Download as a .ACE <
HERE>
Check a Screenshot of it<
HERE>
Check an Alpha Shot of my Mod Editor (Unreleased) <
HERE>