From what I gathered while digging around, the assets by themselves would not be very helpful to most modders; the textures are packed in a very specific way - for instance, ~10 wonders in one enormous texture file. I already made a first cityset to test the waters - and I had to repack the UV or else I'd end up with a texture ~8x bigger than what was really needed. Attaching it here, though I doubt it will be useful to anyone - it's basically a close relative to vanilla Ren Civ 4 cityset; made it simply to appraise the detail level of models and textures one can work with. Also attached is the original texture file, simply to demonstrate the amount of re-packing needed; it is easy to see that at least half of those textures belong to other structures.
So while the assets are actually really easy to extract (it's a Unity game, so extractors/converters of all kinds are readily available) and often have Civ-appropriate polycounts, making anything useful for Civ 4 out of them requires lots of additional effort.