My dad just bought an enclosed trailer that is wired with AC and this is the plug on the outside of the trailer. He is looking for an adapter but this 4 prong plug seems to be uncommon. Is there a reason to keep this plug or should he change it to a more common 3 prong twist lock plug?
I'd stick with what's there. https://smile.amazon.com/dp/B000BQM3GM/ref=twister_B01KRK2G0Y?_encoding=UTF8&psc=1
I did see those wire yourself adapters, didn't know if it mattered that it's not a twist lock style with a seal when used on the outside of a trailer.
Don’t change it. You can’t safely use the ground for your neutral. 4-wire twist locks are available at all home improvement stores. Be sure to use a 10/4SO cord with it.
So that connector is a 30Amp 120/240 volt connection. Line 1, Line 2, Neutral, and Ground. https://www.homedepot.com/p/Relianc...0-Amp-125-250-Volt-Connector-L1430C/202216503 Ideally, it should be protected from rain. Your pic doesn't show enough to tell if the cover is broken off or its mounted in some strange fashion. Normally, you would have this under a rain / dust cover that would protect from falling rain when connected and from dust and water when closed. If you want to be more careful or paranoid, you can get these L14-30 connectors in Marine configurations - watertight with rubber boots and threaded sealing rings. They will run 3-4x the cost of the connector linked above. If you make an adapter, adapt it to a 4 prong RV plug. You should include a dual pole 30A breaker in the adapter, as the 4 prong RV plug is 50A source and you are connecting it to something that can only handle 30A.
If you don’t want to make your own cord, you can buy the generator cords as well: https://www.homedepot.com/p/Reliance-Controls-40-ft-30-Amp-Generator-Power-Cord-PC3040/202216500 Beac made a great point about using a marine shore power plug.
