Something that came to my mind, you might perhaps use for testing is trying different suspension settings, stock, stiff soft and see what kind of differences it makes. I can’t talk much about multiplayers but in addition to curves you might try dirt uphill sections. When I was doing dirt race challenges yesterday for #weekly Forzathon challenges and tested different routes to post as tips, I came to think once again how cars sometimes struggle uphill on dirt, not because of direct lack of power, or constant wheelspin, but it feels like rear of the car was bouncing too much. It’s important to note that I used stock '75 Porsche 911 Carrera, with AWD’s effect isn’t the same. Route with that uphill section is Lakehurst trail if my memory serves me right.
Something you may want to try too, just for curiosity, is testing different suspension types. Leaf suspension of older cars to springs and rather original solution that was used IIRC in both, old Volkswagens and Porsches.
Dirt definitely is different to tarmac, question IMO is rather what sort of things attribute to differences and how it compares to other games and real world. I haven’t played Rally games for a really long time but Colin McRae Rally was really good, didn’t liked the tint in first person view in CMR 2.0 but the best games IMO were Richard Burns Rally and Rally Trophy. Compared to those games FH4 dirt physics feels rather sticky.