It all depends on how much gearing, how fast you want to go etc, for point-to-point positioning the constraints are not as high as for interpolated axis, also you will need to factor in the cutting forces, which if you have a present set up can be measured.
4kw is a liitle over 5hp.
I saw one gantry about this weight that ran on light railway rail and had a rack&pinion for drive. Remember that maximum torque usually occurs at acel and decel. Minimum generally occurs at rest and at-speed.
Al.l
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.