Caster unit is a metal platform or plate welded to a frame with four small wheels, one on each corner. The caster unit is just large enough to fit a tire on the base plate. Four caster units are used to move a car.
They are used in New York City by construction and paving companies to move a car out of harm's way when repaving a street, for example.
The front or rear of the car is lifted and a caster unit is slid under each wheel. A car on four casters can be pushed by one strong person.
It can be difficult to push a castered car over a rough surface. We don't have dirt roads and cobble stone streets are not common in Manhattan but a nice size pothole can bring the move to a halt.
Richard M, NYC