If they just wanted you on property for 4 days why not just offer a $95 discount on the regular 4 day pass? It's more than just that. With a 4 day pass how many people would spend 2 days (or more) at MK and split time at the other parks or not visit them at all? MK is overcrowded. The other parks not so much. Why do you think they charge more for a 1 day ticket to MK?
They could fill out the other parks with rides and attractions to spread crowds but that will take time. Gimmicks like this help artificially spread the crowds. They can't guarantee people will stay in the park a full day, but if they leave they can't hop to another park so the guest is paying for a full day whether they use it or not.
A lot of people might use a 4-Day pass at simply MK and EPCOT ... or maybe they'd stop into TPFKaTD-MGMS and soon TPFKaDHS to ''see the Frozen stuff'' and use FPs at TSMM or ToT.
This thing forces someone to spend a full day in each of the WDW parks in order to pay $50 a day (which is about what I think all the FL parks are worth now).