Brompton seems to be trying to build as much of the bike in-house as possible, and in fact, W B-A talks about this in his book. One of the results of that is Brompton's infamously odd and limited drivetrains.
Brompton doesn't make the drivetrain in-house, the IGH come from Srurmey Archer and the 4s derailleur is made in China (like most components, brakes, rims, saddle, hub...). Only the frame is made and painted in-house.
BTW, I wonder what can be patented in this 4s derailleur ?