The neutral should be bonded to the ground outside in the main panel/disconnect. Four wires should be run from that main to the panel inside the trailer. There should be no connection between ANY of the grounds (green or bare wires) and the neutrals (white wires) inside the trailer panel. The ground wire ran with the feeder should be tied to the trailer grounds as well as the panel box itself. The main neutral should connect to the trailer panel neutral bar containing the circuit neutrals. This neutral bar SHOULD NOT be bonded to the panel enclosure and thus the ground bar.
Remember, just because an electrician claims to be licensed, that does not necessarily mean that he knows what he is doing (I have worked with a number of very code ignorant licensed/certified electricians!). I would suggest having a more knowledgeable professional remedy the problem and them bill the initial electrician for his mistake. This serves two purposes. First of all, this "electrician" needs to be made responsible for the addition costs incurred by the homeowner. Secondly, the electrician needs to learn from his mistake and begin wiring future trailers correctly. All four wired subpanels need to be wired this way so this electrician is most likely miswiring all other such systems. This needs to be stopped (I sure hope this guy does not have his hands in any farm wiring! Not four wiring or four-wiring incorrect are a couple of the big issues regarding that ugly subject known as stray-voltage).
