It's a classic traction circle challenge. The tires (typical street tires) can hold on to about 1.0-1.2 G's of total combined force. If you use all of that up with braking, there's nothing left for turning and vice versa, and the Z4 is particularly sensitive to this. If you come off hard on the accelerator and jam on the brakes you get more weight transition than desired and you either overload the front tires and get understeer, unload the rear and get oversteer, or as it sounds like happened to you there was initial understeer until the front tires got some bite and then when they did the rear was unloaded so it looped on you. As difficult as this sounds to do, you need to allow just a bit of a pause to allow the suspension to recover from the acceleration and then roll onto the brakes. You can brake really, really hard but just don't stomp on the pedal initially. Learning to left foot brake will definitely help once you get the hang of it since you're not having to move your right foot back and forth frenetically between the gas and the brake.
If you download the free Track Addict app to your phone, it tracks G force data and if you use the Race Render app it will overlay a G circle on your video so you can see when and how much force you were generating.
Just some food for thought!
