I have loud speakers in the door trim. I went for some 100mm diameter speakers. It's useful to buy the thinnest ones available as there is not much space between the door trim and the metal work.
The Renault 4 is quite noisy inside so there is no point spending too much money on the sound system. It's probably best to buy a cheap radio and speakers. The speakers made from plastic are better than the ones made from card (less chance of rotting).