Marble machine 4 has been around for two and a half years and it's had a few tweaks to it but this is the first significant revision. This resolves 2 of the larger bug bears, if the steps are a bit tight they don't have enough weight to drop back onto the cam and if you flip the machine over all the steps fall out of the machine. A little flange was added to the bottom of each step, this catches on the sides and stops them from falling out. A ball bearing was embedded in each step, this gives it significantly more weight it drops down on to the cam much more reliably. Kits as always available from my store.