I need a motor that can turn about 100 lbs. at around 4-6 rpms under load.
100lbs is meaningless in this context - for a motor you need torque (turning force) measured in Nm (or sometimes ft-lbs).
Torque requirement is easy to assess for a winch since you multiply the pull-force by the radius of the cable-drum to get the torque on the shaft.
For a vehicle its harder to know the torque without knowing more about transmission, terrain, slope, tyres...
For simply turning a heavy wheel on a bearing you need to know friction losses, but you may need a lot of torque to accelerate the wheel to speed fast enough (and then braking torque to stop it). Torque here is angular-acceleration times moment-of-inertia (plus any friction losses). Moment of inertia is proportional to mass and proportional to radius _squared_.
[ You meant RPM - its already plural "revolutions per minute"