sherline spindle for machine?

I have a bunch of precision parts I want to throw together to make a
small benchtop cnc mill/lathe. I have 3 nema 23 brushless motors for
the x,y and z axis that have a stall torque of 1.5nm and 1000 line
encoders and a 4th nema 34 brushless motor with a stall torque of 4nm
that I plan on using as the spindle motor. This motor can spin at
8000 rpm, so I was thinking of using a 2:1 reduction to a my sherline
head. Would 4000 rpm on the sherline head be pushing things, or is
that acceptable?

The reason I am putting the brushless servo motor on the spindle is
so I can rotate the head 90 degrees and have a lathe with threading
capability. Also, I might be able to do rigid tapping. I believe
this 400 watt nema 34 brushless motor should have enough torque to
act as a spindle motor.

The final machine will have x travel of 12 inches, y travel of 20
inches and z travel of 10 inches. I am using .5" pitch ground
ballscrews for all axis. The table moves in x on a Daedal crossed
roller bearing table while the head moves in y and z on linear rails
and carriages.

Rick