@OP, I would recommend 5 over 4, but I think 5 spoils one for 4. So if you think you might want to play both, then 4. Also 4 complete has some fantastic RP scenarios with it.
Could someone remind me why one might have felt the 3 to 4 transition was too rough? In my memory, it was quite incremental!
Its mostly because when you are used to something and things change, that it sucks. Espescially if you were good at something and all of a sudden you suck.
What i think put me down though was that it turned around my view of strategy games. Strategy games in their basic form are about exponential growth and more is better. In civ3 this was very strongly true. You could not have enough cities or units. Sure, new cities you built have corruption and dont perform as well, but they are in no way detrimental to the empire you already have. Similarly, when you capture enemy cities they might be resisting and totally corrupt, but they were in no way bad for your empire as a whole. That was what i was used to and what i new to exploit to the max in civ3.
Then came civ4. I dont know much details anymore, i didnt try it for long. I do remember that all of a sudden more cities and conquering cities could actually be detrimental to your civ. It turned my whole world upside down, the basics of all strategy games were no longer true in their simple form.
It is for the same reason that i thought warcraft 3 sucked after being a 6 year full time starcraft veteran. The unit upkeep levels in warcraft 3 in my eyes were simply a punishment for playing well. (the difference is, i still think warcraft 3 sucks, but civ5 has taken me over)
That was the most important part, i dont remember but i also think there was something i didnt like about combat, something that was very unline civ3s stacks of 20 horsemen owning everything.
Now, many years later, i started playing civ5, obviously the same is true in civ5. But now i have changed as a gamer. I became more of a casual gamer, didnt expect to use my civ3 skills to own civ5, instead just started playing to see a pretty game with nice world wonders to build like every noob
(sure, i have already lost several nights of sleep and not playing so casual anymore, but that was not my intention
)