I like the idea of all the parks being so close together. The new park would be so far away and it would parkhopping rather difficult. I don't think they will need a 4th park for a while. The current parks could use a bit more work in some areas but that is just me.
The new park would be shorter than the distance between MK''s Transportation Center and Epcot. But they aren't stopping with additions to the regular parks either, Nintendo, Fast & Furiors, Rumored updates/additions to Marvel Island, More Harry Potter stuff, Jurassic Park stuff, and Toon Lagoon all are up for updates. (Over time, not all at once)