4 ply . 10 is to STIFF. Will Bounce you rite out of the Machine 
Unless you are going to be using your machine in an environment that will be very rough on tires. IE large thorns, sharp rocks or anything that is going to puncture or cut your tires. 10 ply would probably be able to be used at 0 psi and still function well if it was to be compromised. Otherwise 4 ply should do it.
