I have to admit, I never really played SEIV beyond one crack at the demo... maybe because making my own ship graphics was too hard. The graphics got better, my skills hadn't
I think the only thing that put me off in SEIII was as the game moved into the later stages you'd have a LOT of planets. I'd either have to tediously cycle through them, ensuring they were upgrading to the latest facilities (and since you couldn't actually upgade, destroying the old and queueing the new) so I'd turn if over to the AI Minsters (which was their purpose, was it not) but they wouldn't delete old buildings to construct newer copies. I think after a while of that I got put off... that and the fact that any race that was peaceful always utterly dominated anyone else
I ran across TDM once, though I'd forgotten the name. The AI is an element of the game that greatly interests me. I was put off by GC's simplicity after the options I had in SEIII, but the AI in SEIII was lacking in certain areas (once point defence to shoot down the AI missiles, then once I'd gone high enough in lasers I could dominate their fleets, I'd just leave the battles to the AI... tactical combat was pretty boring IMO). I much prefer to having turn after turn rather than waiting days for the next turn, though after playing with a friend I have to admit, there was a certain thril to it
How much does TDM improve on the AI? How far does it raise the bar?