Also, let's be sure we're talking apples to apples. You're saying you used to buy 3 day base tickets to Magic Kingdom (aka "MK") and are now looking at 4 days tickets to "MK". Are you only planning to visit that one single park? Just want to clarify, because though uncommon I've heard people confuse MK with Walt Disney World. If you're only visiting the one park, I'd suggest that you're missing out on SO much. I'd look at doing more than just one.
As far as MK itself goes, as others said it depends on what "everything" means. If you're talking every single shop, every single attraction, every single show/performance that is a lot of things. If you had a good plan, I think you could do it in 4 days if you spent a good amount of time in the park each day. It would probably even give you enough time to stop and smell the roses some.
But again, you may want to think about doing a few other parks too.