Nothing yet has gone wrong, except some minor repairs (exhaust, brakes, changing oil, tires ....) after 100.000 km.
It is very good car for its class. I am very sorry it isn't possible to buy a new one. It is cheap to maintain, and it is a very economic car.
The Renault 4 is lovely car with unique shape, little bit funny car possessed by local Don Juan. I heard (is it true or not) that, for example, hood of Renault 4 is same price like hood of Mercedes or BMW. Also I heard that this car is not good because things like mirrors, handle of gear lever...,can be easily broken and it is another expense for owner. Evil tongues said that floor of the car can be eaten off by rust and passengers can drop out!
The original French parts are quite expensive, but they're still worth using, since Italian parts are of lower quality and tend not to fit properly.
True, rust can be a problem, but I've never heard of anyone falling out of it. Nobody's fallen out of mine, and I've managed to squeeze eight people into it once. Got a ticket for doing it, too. :)
I do not really agree with your comment: the hood of the R4 has got the same price than Mercedes, or BMW's. I am living in Germany and if you look for spare parts for BMW and Mercedes they are REALLY expensive. As for the mirrors and so on... they can always break, but not easily, though remember you DO NOT NEED special tool to replace them :-)
You can't fall out of it through the floor because the floor is a part of the frame. That's malicious.