Originally Posted by Norse

I would go down on your diff oils being you are on dirt. Thus as well run heavier in the front diff and lighter in the rear which should help out. Try 7kf 3kr or 7k 5k and I have heard people running 5kf 3kr. Have you tried your clicker both locked down as well as dialed back?
Running Saddle packs or bricks sets the back end down. Yes it's added weight but that helps as well as the added weight covering more of the chassis rear surface area. Try a little more kick up with your inserts being set to add more neg caster. 2.5. You could try more droop in all your shocks. I have the thinnest limiters inside( more so for a little added protection on the bottom side of the shock piston) thus they are so thin they really aren't limiting my droop. More droop in the front will tend to shift weight towards the backend when you apply power.

You may want to drop your shock oils down a tad being -2.5. I believe AE is a little lighter than other brands.

Also the car needs to a good amount of packs run through it to breaking the diffs and belts.

If you have any drag break set on your ESC dial it way back or even totally off.

I hope this helps and I am sure some of the other YZ4 owners will share their advice.