Sorry to show up late to the party.
I would think that even if the machine weights 4000lbs all of the weight is not on one pair of skates. So lets say I use a pair of these on the heavy end and a pallet jack on the other(for steering) it should be OK. The other thing I was trying to solve is 4 bearings per skate vs 8, you should have less rolling resistance with 4 vs 8. I am trying to build a set just like yours and I was thinking of using 4 bearing per skate like you, maybe put some plastic matting on top or a piece of wood to stop slip. That would be only 250lbs per bearing. I think I could live with that. I think I will use steel though(because thats all I have) and maybe a piece of 4x4 wood wider than the base of the lathe so I can get more stability. Then I can bolt the lathe to the 4x4.
I hope I am not infringing on you copyrights and thanks for posting pics!