All four straps are needed to secure the wheelchair to the floor of a vehicle. This method is most often used in transit buses, paratransit buses, school buses and in personally-owned vans. It works well in these settings because the straps work with many types and styles of wheelchairs. The standard for testing the strength of the securement of the wheelchair to the floor of the vehicle with straps or a docking system is comparable to the standard used for testing the attachment of a car or bus seat to the frame of the vehicle.
The objective of 4-point securement is to attach the straps to the wheelchair frame just below the combined center of mass of the wheelchair and it's occupant. The angles from the floor to the wheelchair are carefully defined within the standard so that they stabilize the chair and prevent tipping, rotation and forward movement.
It is important to use all 4 straps. Some bus drivers, transit aids or passengers might consider attaching only the straps that are the easiest to reach. Not using all 4 straps places too much of a load on the straps that are used and may also lead to tipping.
The following YouTube videos show various scenarios such as failing to use securement, using only one front and one rear strap, or attaching the tiedowns to improper wheelchair locations (such as rear wheels). The final video shows the use of all 4 straps.