350 RPM with a load of 4 times 2 pounds.
is that foot pounds ?
spinning without vibration at 350 rpm is all about balance. your chain will be noisy and contribute to the overall system vibration.
you may need to have a smaller drive and a larger driven pulley and run the motor at much higher speeds.
the steppers properties are such that they deliver high torque at low speed.
also, due to the design,there is an upper practical limit for steppers. you cannot get large motors. the force to make them step becomes almost as great as the resistance from the magnets.
the industrial servo (really just a DC motor with controls) can spin very fast and deliver less torque. the servo is designed to be reduced by pulleys and such.
without knowing if you have a 2 pound weight on a rolling cart or 2 foot pounds, the actual motor spec cannot be determined.
I think that as you define the problem, the answer will present itself.