Short answer is I don't think this can be done due to the fact that people book in low season for stays in high season and vice versa. And you want the max occupancy to display based on the stay dates and not based on the time of booking...
One potential work-around would be to set up two listings -- one advertising max 2 people and the other advertising 4 max people. Then block off the high season on the low pax listing and block off low season on the high pax listing. Of course this won't work either if you are on subscription because you'll need to pay for 2 subs. But if you are on Pay-Per-Book, it may be an option for you.
All that said, I don't think the juice is worth the squeeze since you say you will allow 3 in low season and 4 in high... for one person (likely a child) it may not be worth it. Just a thought.
Why don't you set your rates starting at 2 people? Then charge an additional fee for each additional guest. That is what we do because we want to keep our rates lower for honeymoon couples. It would be good to know why you want a certain number off season verses high season. Is it because of wear and tear? Or is because you are more likely to book to larger groups during the high season? Or is because it is easier to clean after just two people? You could also charge a lower cleaning fee if there is only one bed used.
We used to book to larger groups up to 8 people. Even though we could sleep that many and had the beds to do so, it wasn't desirable for wear and tear as well as cleaning. So when the county set new rules of 2 persons per bedroom regardless of age, we decided to go that route even though we were grandfathered in and could take more than just 2 persons per bedroom. We find the fewer number of people is definitely more desirable. It is so much easier.
More people travel with children in the high season, summer. So 4 is a good number.
But in the quieter times Oct - May, I really enjoy 1 to 2 guests. The rental is below me and I love the more laid back guests.
I do have a two bedroom as well below me, and used to be able to encourage 4 guests to take the two bedroom instead and would off a bit of a discount to do so.
But with the new booking system, my ability to sell, and offer, has been limited with not being able to speak with them before booking.
All great ideas.
I think bonesxxx has come up with a good solution. Even if you have a subscription, if you start a new listing, you could have that set up for PPB. The downside to having two listings for the same property is that your reviews will be "split" between the two listings. Also, it may be confusing to travelers who search without dates to see two listings (with different maximum occupancy requirements) for the same property. There's a PM in my area who has two and sometimes three listings for the same property and, personally, I find it confusing as a traveler (and annoying as an owner because the multiple versions of their listings appear above my listing ).
I'm just curious. I think some vacation rental owners are willing to relax some of their rental requirements to get more bookings in the off-season. Why do you want your maximum occupancy to be greater in high season, and lower in off-season?