You've several options:
The best one is to buy some P43T 60/55 watt halogen headlamps, and then buy some selective yellow glass caps and slide them over the bulbs. These will fit into a Renault 4's headlamp unit, giving that characteristic "French" look to the car, and with bright halogen lighting as a bonus!
Another option is to shop around on E-Bay (other internet auction sites are available) for some "old school" selective yellow 45/40 watt bulbs and fit them to the car. These turn up from time to time.
Painting the headlamp glass yellow is another option, but not, in my opinion, to be recommended.....
The fitting of "selective" yellow headlamps is legal (in the United Kingdom-I cannot speak for other counties)