We need to know the travel of each axis, and the pitch of the 16mm screws to work out if they are suitable. The main limiting factor is in theory the speed at which the screw starts whipping - critical speed.

The only 4Nm Nema23 motors I know of are these which are the same part number:

http://www.cnc4you.co.uk/index.php?r...&product_id=72

Their inductance is nice and low. The rotor inertia isn't stated...
I'm intending to get a few of them for my router so that I can put the ones from the router back on my mill. Not sure how much difference it'll make over the 3nm. I'll be interested to see what they're like.