If no net force acts on a particle, then it is possible to select a set of reference frames, called inertial reference frames, observed from which the particle moves without any change in velocity.
This law is often simplified into the sentence "An object will
stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force".
This means that in the absence of friction (your tires on pavement, for example) once you get your 4wd moving, the Good Lord and the Laws of Physics don't give you a glimmer of hope of stopping unless you can impart an external unbalanced force (hitting a dry patch, deeper snow, or a bridge abutment.)
For all you deniers out there; this one isn't just a Theory. It's the Law.