Is it just me, or does gold and upgrades take on a new dynamic in CivIV than in CivIII?

In CivIII, you could build Leonardo's Workshop, which would cut upgrade costs by 50%. This, coupled with the fact that you did not have to change your research slider in order to be making money right out of the gates, made for manageable unit upgrade costs.

In CivIV, you must leave your research slider at 100% in the beginning or you will begin to fall behind in the tech race. Therefore, you are not making any money for the first few turns, which can hamper you. Furthermore, upgrade costs are very high, meaning that you can only afford to upgrade a few units at a time, which means you are constantly upgrading.

In general, I don't. That's not to say I never upgrade, just that I don't upgrade unless there's a good reason to. Some good reasons I can think of off the top of my head: 1) The old unit is highly promoted or has promotions the unit I'm upgrading to can't get (city raider for riflemen for example.) 2) I'm being attacked and I've got outdated units in my threatened cities. 3) I've just learned a new military tech and I'd like to attack immediately while I've got a tech advantage rather than waiting 10-20 turns to build a new army.