There are 4 pin to 7 pin adapters that work, as long as the vehicle has the remaining wires terminated at the rear of the vehicle, as my Ford Expedition does. Bill's statement is correct that you cannot make 7 wires out of 4 if the vehicle only has 4 wires.
Fortunately many vehicles come with 7 pin wiring harnesses (or 6 plus a caveat for mine). When owners choose a tow package they specify if they want 4 or 7 pin connectors in the back.
The caveat on my Expedition is that the 12v power from the battery terminates at a connector under the left passenger door. You have to split the wiring cover and splice a wire into the connector and run this wire to the power wire on the 7 pin connector. The only other option was to replace the wiring harness from this connector to the rear of the vehicle, including all the rear lighting connectors (that wasn't going to happen!). Why Ford did this, versus running all 7 wires to the rear, is a mystery only they can explain.
Owners should search the Internet for their vehicle wiring if a different connector is needed; there are a lot of sites that provide details such as I needed for the Ford.
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