You may have to make up your own pig tail adapter, using a male and female plug and a length of multiple wire power cord. Your four pin plug only has brake, running lights, turn indicators and a ground wire. Not all the 7 pins in you trailer are being used. You will have to determine which ones are, and what their functions include. You will not have a hot wire for interior lights, back up lights, nor a braking circuit coming from your tow vehicle.

Gard

Brenderup trailers use an inertia braking system, not electrically controlled by the towing vehicle

If I understand correctly, this should work;  http://www.etrailer.com/pc-A~30717.htm 

But I would match the trailer plug to the truck, or wire the truck for a standard 7 blade rv socket.  Im not a big fan of adapters other than for temporary use.

What are you driving?  There are a lot of plug and play parts available.

Or for that matter change the whole thing out for an EZ Connector..............

Wally Mart sells converters check in the towing section.  I convert my four plug poop wagon up to the seven pin on the truck.  I have one on my bench that goes the other way.  Easiest thing to do so you don't have to change any factory set ups.

Just found this write up...

The inertia brake system doesn't need a trailer brake controller.  These are the standard brake system in Europe. The brakes are activated by a surge type coupler, but instead of using hydraulics like a boat trailer, the coupler pulls a rod attached to all 4 wheels by cable. This is speed and load sensitive. The brake drums and bearings are maintenance free. Twin leading shoes in each drum, are bi-directional,  when you back up there is a 3% drag factor. The parking brake will turn the brake cam over center and lock the wheels from turning in either direction. Having a parking brake is great. there are none of those brake magnates in the wheels. Great braking, it all seems to simple, but it works. No brake magnates, no break away battery, no trailer brake controller, no sway, nobody else does all that.

Correctomundo... BlazingCreekBar, all mechanical, no fluid, no electricity, just cables, chains, rods and a spring or two, keeping it simple.

http://horsetrailerworld.com/Home/contribute/Homepage/Mrtruckbrenderup/mrtruck.asp

http://www.brenderuprealtrailers.com/news.htm

Very cool rigs.  

A friend of ours has one attached to her Hybrid SUV.  I like the trailer.  I worry sometimes about it and the tow vehicle not being heavy enough to compensate an issue.  But if your careful you will stay safe.