I would expect the 2-pole motor to be your best bet - synchronous speed is 3000rpm @ 50Hz, but clearly less in reality due to slip. I think running a 4-pole motor, which is only designed for about 1500rpm, at 4000rpm is asking for trouble. At the very least the bearings are going to want replacing, and I wouldn't be surprised if the rotor is not strong enough (centripetal force and all that). With a 2-pole you're not exceeding it much, no doubt a good motor will be safe at about 30% more as they are tested at a lot higher.

When the motor is operated above the rated speed the voltage applied to the motor should be reduced proportionately so that the power rating is not exceeded. This means that at higher speed the torque will be reduced.

The problem with a 2-pole motor at only 1000rpm is it might have lower torque (I'm not sure)... if it needs to be constant (or high) then ideally you would need vector current control, which the Chinese VFD does not do. If the VFD is set for constant V/f ratio the motor should output the same torque from 1000-3000rpm.

A lot depends on what you're using the motor for? If it's not going to be operated anywhere near it's rated power then I should think you'll be fine to just use the Chinese VFD.

I have used Chinese VFD with my Colchester lathe, which is a standard army issue 4 pole induction motor. It works perfectly well - so far I've only dared run it up to 60Hz.

Hope that helps ... I'm not supposed to know about this untill 3rd year