I have to go with all four, too. It's a pain having a girl go into heat like Twiggy is right now. She has to be completely isolated, which she HATES and the boys start humping anything that moves including each other and the poor cat even though almost all are neutered. And I get some agression between them too.
I would also have all 4 done. It's a pain when girls are in season, you might think you have done everything to stop the girls being mated (especially if you ave boys) but don't under-estimate the nature. They are a lot stronger and my girls got mated (even by the neutered boys (yes!!) even may be there's no pregnancy. )
For the long term health reasons and peace in mind, get both boys and girls done despite there are lots of tales tales around.