If you're running out of ideas for the fourth video, I have some suggestions:
1) A figure-eight donut. Find something like the digger in the Parking Lot. Do a donut around the scoop, then change direction and do a lap around the cabin. Change direction again and go around the scoop, and then the cabin again. Rinse, lather, repeat.
2) Drive on two wheels. I'm surprised neither Ken Block nor Codemasters implemented this. There's a few ramps around the compound that you can use to do it, though I've never succeeded in doing it deliberately myself. Get yourself up on two wheels and drive as far as you can.
3) Race around the scaffolding in the Power Station. Climb up to the top level of the Power Station from the Parking Lot side (you should hit the jump on the unprotected side). Get back down as fast as you can without hitting anything. It's very narrow, and so very diffcult.
4) Drift across the tops of the containers without falling off, either in the Parking Lot or the Waterfront. I've found this one to be practically impossible, but I can see how you would do it - you just have to start the drift at the right moment and control the throttle enough that you can pull yourself out of it in time.
Another gymkhana arena would be nice, but as far as I know, there are no plans for any more to be released. The only remaining circuits to be made available as DLC are a series of rallycross stages in Shibuya. However, I see the Compound you're making these videos in is different to the Compound I have (I haven't played the game ina week or more), because there are barriers placed around it that I imagine came from a patch. Codemasters might keep updating the Battersea Complex that way.