they were not strategic

That's not entirely true. While Civ in general and IV in particular will probably never be known for deep combat, IV did have some interesting things, and I would argue that most of its flaws are the same as the flaws in V's system - not that the system itself is bad, but simply that the AI doesn't understand it.

First, stack composition was, in theory, important. Siege weapons were a huge limiting factor on stacks of doom, because they hit every unit in the stack, not just the top one. Mounted units, on the other hand, could do the same thing, but only against siege weapons. Difference is, Mounted units could kill siege weapons through collateral damage. So siege weapons crush almost everything but get rapidly destroyed by mounted units. So of course, if you want to defend against Mounted units, you've gotta get some anti-cav units in there. Those were usually melee units, so you'd need to get some anti-melee units in there as well to protect those anti-cav units against other anti-melee units.

The reason I say "in theory" is because the AI never grasped the insane power of siege weapons, so all of this became more or less moot in single player.

There were also strategies like "forking" - positioning your army in such a way that it can threaten two or more enemy cities, forcing them to divide their forces until they know which one you're going for, effectively cutting their army in half.

It actually annoys me a bit, because Civ IV had the potential to have a great combat system, with a lot of depth and possible strategies - but this was more or less eliminated by the fact that the AI doesn't understand any of it.

As much as I can say the same thing about Civ V's system - that it has the potential to be strategic and fun but all of that is ruined by the terrible AI - V's is a bit better than IV, in that regard.