Buy only h4s made in Germany. Not expensive either. You will find the country of manufacture on the package. I would also do a quick check of the battery voltage at idle.

If you want long lifespan German manufactured H4 bulbs give you the best chance. Suppliers are well aware of this. As are many h4bulb buyers. I carry a spare Chinese bulb that costs a dollar delivered out of China in each car. Just in case I lose a bulb on a trip. I do not use Chinese bulbs because of their know short life charactaristics.

The way this works in the real world. If you have a spare bulb chances decrease of one burning out. Do not ask me why or how. I really do not believe that is a factor. At the same time I would not bet against it.

If you had a high resistance anywhere in the bulbs supply current it would increase the bulbs lifespan. Plus in most cases the light would be dimmer in comparison to the other side.