I'm not a scientist, but wouldn't opening less parks mean that the amount of people that would be capable and willing to visit 4 parks would be crammed into less parks? I'd prefer crowds be more spread out personally.
Also, just going by Disney's past behavior, I don't see why they wouldn't just open all 4 parks with half of the attractions (especially shows) closed, rather than open half the parks. They had no problem charging full price when DHS had only 3 or 4 rides the last couple years, I think they can open EPCOT without 75% of World Showcase offerings at the start.
I just don't see WDW opening without being able to offer "4-Park" passes. They're not set up for that. The parks are either open, or not. I don't see any in between. My take anyway.