First note that Civ4 is quite old, thus you do not need any fancy hardware to get it running.
It is a 32 bit application, meaning that it can not use more than 4GiB of RAM anyway. I have heard that Apple stopped supporting 32 bit applications, so that may be the reason it stopped working? Downgrading to an earlier OS might solve your problem. If you want to do this, I would recommend you do some thorough research.
It is not hard to run for any moderately new graphics card, including on-board ones.
It is entirely single-threaded, so the number of cores does not matter if it is greater than two. I say two because I read somewhere that it can use a different core for graphics related stuff, but I may be wrong. For the game decisions themself, it definitely uses only one core.
Basically the only parameter of a modern system that still matters is CPU frequency.
Back in 2005 it ran better on Intel CPUs/ Nvidia GPUs than AMD ones, and while this is maybe still a little true today there is little difference. I only have the comparison between the few systems I have run as data.
As for the OS, it was built solely for Windows and will thus run there without much of a setup. Running it via wine on Linux now works just as well, with some people even reporting slightly better performance, but comes with more installation overhead. On iOS I have no idea besides the above.