It's definitely still worth buying Civ4. Arguably the best game in the series, and it runs great on modern systems, no fiddling around required. And that's even before you get into community mods; IIRC Civ4 has the greatest modding potential of any of the Civ games, and it's been out long enough for a lot of potential to be realized.
I had two older laptops running XP, but the newer one had its motherboard die last year. The older one still works. But I don't actually need them for Civ4.
My main computer still has a 27-year-old internal floppy drive. The trick is to buy a USB to floppy adapter device, and a USB-header-to-single-USB-port adapter, then you can slide a 3.5" floppy drive into a 3.5 or 5.25 inch bay, and connect it up to your motherboard's USB headers internally, making it fully integrated into your desktop. Probably won't help much for storing Civ4 saves, but if you need some authentic floppy drive sounds every so often, or just to read data from old floppies, it works well.