The wear out indicator on the tire sits at 1.6mm, so at 4mm you would be more than double over it. And so you should, at 3mm you are on the limit if there is water on the road, and anything under 3mm is inviting aquaplaning and loosing control. And no electronic aid will work when there is no grip, so don't count on the ESP or ABS to help you when you aquaplane, it will probably make it worse.
If it was a one time occurance, maybe there was something on the road, water, grease, oil, dirt, sand etc. You ran over the dirty patch with the wheels turned so they lost grip and you had understeer, then the front wheels got to the clean road and gripped well but now the back wheels were on the dirty patch and they had no grip. So front wheel grip but no back wheel grip would be snap oversteer if you had the wheels turned (and you did).
Find another roundabout or a wide street and try it out again. If the car feels fine than it was the road there. If it doesn't start by checking tire pressures, then shocks and alignment (although the standard suspension has built in camber on the front (and i think castor as well) so you can only adjust the toe, and at the back they are all fixed, so not much to go wrong.
