Win10 is a lot more compatible with old disc based games than many people realise. The vast majority of games from 2002 onward,
and many as far back as '98 will run on Win10. Games older than 2002 are more likely to require special fixes though.
Anyway, I am confident that Civ4 will run on Win10 64bit, but with the disc based version of the game there is an elephant in the room
that was created by MS - DRM Blocking.
This is one of hundreds of games that use SafeDisk copy protection and MS built a 'security feature' into Win10 that blocks all
disc base games that use SafeDisk from launching. More details about this and a few other DRM issues in my post at ---this thread---
- Short story - you will need to use a noCD fix\hack to run this and any other disc based game that uses SafeDisk.
(link is to fix for main game).... megagames.com/civilization-iv
It would be best to patch the game to the highest level available and then use the fix that matches the patch version.
"Steam isn't even working properly anymore"
The Steam client can be repaired or reinstalled easily enough.
They have instructions for repairing in Steam support > Faqs.
Good Old Games have the Complete ed. It's a bit over priced atm (full price), but as with all the game shops they have frequent 'sales'.
with 50>75% off the full price.
https://www.gog.com/game/sid_meiers_civilization_iv_the_complete_edition