I would say after 4 years, my abused Dahon Piccolo was suffering from lose joints. Then again, that bike was attacked by bike crooks trying to steal it. A heavy rider who abuses any bike can break parts, stems etc.
The trick is to take care of your folding bike and treat it as gold. You'll get many years out of it.
By the way, even though my Piccolo was abused, it still had many years of service left.