So, having now played FH4 on my new Steam copy, having linked it to an Xbox account that had a lot of progress, I can list the ways in which the existing Xbox account affected the experience.
Credits, cars, skill points, forzathon points are all not carried across.
Unspent backstage passes from an Xbox account are available to spend on a linked Steam account. I assume that spending them makes them unavailable in the Xbox account.
Unclaimed ranked season rewards from the Xbox account were able to be claimed on the Steam account. However, while I got the ranked rewards, the rank itself hasn't carried across, so I need to complete the process of getting ranked from the start again, including doing unranked racing first, to be able to get the playlist % for online adventure.
Anything that is communicated in messages that hasn't been claimed on the Xbox account can be claimed on the Steam account, such as gift cars, payouts from tunes etc.
For leaderboards, it recognises the Xbox performances as being mine, but gives a PB message when I beat the best I've done on the Steam account. E.g. I did 303m for a danger sign and it said it was a PB, but then the thing it pops up where it shows the nearest friend's score against yours, it showed my Xbox performance of 311m. A similar thing happens with discovery of areas and roads. It will say I've now discovered e.g. 30 roads on the Steam account, but then show 531 roads discovered in the popup thing that compares against friends.
So it's a bit of a mix of some things working as you'd want everything to work, e.g. the backstage passes, where it looks like the number you have is shared between the two accounts, and spending on either account removes it from both accounts, and other things such as credits not working that way.
Edit: I'll add any more information that crops up below:
Re ranked racing, my rank has actually carried across. It lost the fact that I had qualified for ranked, so I had to repeat the unranked racing to get to tier 3. But having done that, I only had to do a single ranked adventure to complete the playlist item, and my rating carried over from where it was on the Xbox version.
Tunes are interesting. Tunes that I've favourited are still listed in my favourites. Tunes that I had only downloaded are no longer listed in "my tunes". Tunes that I had shared have to be found by searching for my own name as the creator, but when I download and install them, it knows they're mine, and allows me to edit the settings.