If you have the correct holes for four blades, you can run four blades. That would require a bit more engine power, which you probably have, and might give a slightly finer finish to the lawn or alternatively allow a bit more forward speed for the same finish. On the other hand, when you mess up and hit a water pipe, you'll ruin four blades instead of just two, and you'll have to sharpen and maintain four blades all the time.
I suspect it is mainly a matter of what works for you: how many water pipes do you hit, and how annoyed are you when you have to replace four blades? Four blades would probably lengthen the time between blade re-sharpenings or replacements if you don't hit water pipes.
