Getting closer to my goal of 10 seconds.
The images at the start from left to right are Enrico Rastelli (1920s), Leon Rabello (1936) and Paolo Bedini (1943).
The machine learning shows a genetic algorithm evolving a multi-layer perceptron neural network controller to perform the balance in 2 dimensions. A 2D rigid-body physics engine was used for the simulation. The performance of each trial was assessed based on the time balanced and the average 'uprightness' of the stack over the duration of the balance.
http://www.mathworks.co.uk/matlabcentral/fileexchange/36911-machine-learning-balancing-a-stack-of-balls
I don't think 4 is really doable for a human - way too many degrees of freedom.
Same video but with music that I couldn't upload here http://juggling.tv/12121
