Crap.
The UI is far from being "smooth" in SE4, with the missing of the right-click and all.
All in all, SE5 is the better game, because
- the AI is actually using scripts, not simple to-do lists
- those (and many more) scripts are accessible to modders
Do not play the demo, use Kwoks Balance Mod and watch in awe when the AIs actually take some sensible decisions!
(What you'll never see in SE4, as the AI does not really "decide" anything there ...)
What killed the SE series was not SE5 in itself, but the absolutely dumb idea to sell it to Strategy First:
They won't fund the still needed patch (and come one, no game is really finished, ever), they don't even react to email and the support form on their website does not work
Without that last patch, there are some bugs with show-stopper potential, which cannot be worked around completely.
Nevertheless, I've switched to SE5, and only go back to SE4 very seldomly ... and I always find its not worth it, no matter which mod I use ...