How to run a 6v 4amp motor at slower speeds for long periods of time (2 hrs)?
Reducing input voltage can overheat the motor, glaze brushes, or lead to the motor stalling which then burns the armature out.

Gear it down (cogs, chains + sprockets, belts, etc...) and run it from the correct voltage from a suitable power supply - 4A current rating would probably be draw under load so you might get away with a lower-current PSU in your application (especially when geared down) as long as it's protected against overload.