I had the original 2 wheel shuttle,and had no problems with it at all,although a lot did.
I was tempted to do the conversion on it myself,boss wheels etc,but seeing the new 4 wheel one,its been strengthened,rear wheels are on an axle and it feels solid.Rock solid.
I had the usual mountain of gear on it at the weekend and its a joy to use,was on dry but rutted ground and it flew over it.Im well chuffed with it and can see it lasting a lot longer than my original would of.
Only downside is the rear wheels are fixed on the axle so take up a bit more space in the motor.