Jimmy, each of the four stories needs to meet Medium’s distribution standards, so that’s a hurdle every article needs to clear before it can be counted against the quota.
Otherwise, there aren’t any restrictions on topics. I think it’s a nice idea. It means that, after I’ve published my fourth article for the month, I can take more risks with the fifth and subsequent articles. No more pressure to come up with another listicle to bump up my earnings for the month! In fact, I might shoot for the moon by writing a 30-minute deep dive into a topic that interests me. Why not? After all, if nobody reads it, I still get paid my minimum guarantee amount. But if it turns out to be an unexpected hit, I reap the rewards.