If you want more robust systems/quadrants/etc. without changing any of the techs and such, you will want to check out
FQM Deluxe. It can easily be combined with TDM to get both better AIs and better maps.
This link explicitly takes you to a listing of the mods for SE4:
http://www.malfador.com/se4mods.html
[ August 03, 2004, 15:28: Message edited by: Imperator Fyron ]