in my opinion the greatest difference between galciv2 and civ4 is the deepness of gameplay.
civ4 is a very good ggame and truly deserve the award of game of the year, but in terms of gameplay it is a lot dumbed down just to make the game more commercial:
-intelligence is a joke
-diplomacy is a bit rigid
-combat system is totally irrealistic but very well balanced
-civilizations aren't so much distinguished
in terms of deepness the game lacks of atmosphere, wonder videos aren't so good and there aren't casual events.
while there are these problems the game rocks in all other aspects.
i haven't galciv2 but played galciv 1 and i'm sure that the game will be a lot complex and deep than civ4, but probably will lack some strong points of firaxis game.
summarizing if you are a mature player or you are looking for a depth gaming experience go for galciv2 if you are looking for a very good TBS game but more simple and commercial go for civ4. i will have both and i will surely take civ4 on the shelf as soon as galciv2 arrive at my home because i'm looking for a more depth gaming experience.
anyway don't expect that galciv2 will have a better review than civ4, in my opinion reviewers weren't so honest because u can't do 9.4 to a game that is incomplete and was unstable on a lot of computers (no review said it). after 1.52 patch civ4 deserved a 9.0 or 9.1 but only after that patch. probably galciv2 will receive a 8.8 or 8.9 because reviewers will say that there isn't multiplayer or other minor problems. so don't trust totally in reviews