If you need 4000 you want the two pole motor but you will suffer at lower speeds.
VFD's are brilliant pieces of kit but unlike gearing they can't give more torque in fact ignoring all the bullshit they don't actually give the torque they say as they fudge the figures.

Can you get a two speed reduction in the design some way ?

Yes the motors were rewound for 200 hz operation, done 8 so far and all but one have had no problems running 8 hours per day on 2 and 3 head large routers. One had a problem after they bent the spindle because of a crash.