I'm personally not a YC founder, but I have gone through another high-profile accelerator.

In the vast majority of cases, the "rules" are just a starting guideline. There are always exceptions.

Just like there have been (rare) cases of single founders, the opposite of four founders is likely an option.

The one thing that matters is the same thing that has always mattered. Are you rockstars? YC - like others - invest in people.

If you're an amazing set of people and work great together on an idea that you've been working on for two years, but all means apply. You literally have nothing to lose.